Frequently in my writing I've run into the question of how much description I need. I've seen this question before on the message boards, but I thought I'd discuss it a little since it's currently relevant to a couple of the stories I'm writing. [True even of the story I'm currently working on! For "The Liars," I'm actually finding myself going back and adding a few things.]
My general rule for description (of people or places) is that you need to stick with the rule of relevance: if it's relevant, describe. If it isn't, don't. It sounds simple, but evaluating the degree of relevance in any location is where the tricky part starts. There are three big kinds of criteria I generally use to assess this: point of view criteria, plot criteria, and story criteria.
Point of view criteria are my first concern. I consider the mental state of my protagonist and decide whether it allows them any contemplative time to look at themselves, others, or their surroundings. First impressions are huge deal for me in this context. What, I ask myself, does this person notice when they see X for the first time? If they are in a place where they can be held spellbound and simply observe, they'll probably see a lot. If they're in a fight or in a big hurry, they probably won't notice nearly as much, and I'll be looking for some key characteristics of a person or location that will help it be recognizable in the reader's mind if it reappears. I also look out for opportunities for a character to get things wrong on first impression, and pick up the superficial aspects of something in a way that will allow for a change in that person's opinion later when they get a closer look. I also keep in mind my general parameters for the character's mental state to see how to approach the description - as in my last post, when I talked about using negatively judgmental words in initial descriptions for the character Nekantor.
Plot criteria I've already mentioned a little above when I talk about fighting or being in a hurry. Depending on what's going on, you may not have time to do much describing - and if you have your character slow down in the middle of a battle to the death to notice the clothes that his opponent is wearing, it will seem ridiculous.
I find both point of view criteria and plot criteria easy to keep track of in the moment of writing. Harder for me is keeping track of the third type of criteria: story criteria.
Story criteria are things like, "we're early on in the story and if we don't have some description here, people will feel disoriented." Story criteria are tricky because they can actually work directly against one's instincts in the point of view and plot areas. In some cases, story criteria will give you a good reason to change your plot, to put your character intentionally in a position where some observation is possible.
We're all familiar with stories that place their protagonists in a high vantage point or in front of a mirror in order to allow for description of the setting or the character themselves. Be careful with this. If it takes you away from your main conflict, it may not be a good idea. Push yourself to create opportunities for description that have more subtlety, and make sure not to ignore the effect that vantage or mirror scenes have on your character - vantage scenes tend to make that person seem more contemplative in general, while mirror scenes can make them seem vain. The story need for description isn't enough to justify creating those scenes in and of itself; you need to look to bolster their relevance in other ways.
In "At Cross Purposes," (Analog Jan/Feb 2011) I added an extra paragraph of description when my protagonist first meets the aliens. Why? Because first readers thought I made the aliens too much like Earth otters. It was a good point. My stories are complex, and I'm always trying to keep lots of balls in the air, so I missed that one on first draft. Fortunately, my main character has a penchant for wry observation, so I got to play with first/second impression in two paragraphs that immediately followed one another. I had her think, "Otters!" and then go, "But wait a minute..." and describe a bit. There was room in the plot for it, and it was appropriate to her character. And now I've fixed the problem of the aliens being alien in physiology, which is of course terribly important!
Another example comes from the novel I'm working on, and involves a question of orientation in the world (another story criterion). I got to a certain point and realized that I hadn't established that servants to the nobility can be either male or female - and males can work for females, and vice versa. With the way I approach the story, I don't have the option of just telling the reader this. So I went back over the material I had and looked at the first instances of seeing servants. In the first chapter, my main character sees two different girls, each of whom has a servant/bodyguard. One of the servants becomes a larger character later, and he is male, but the other one was unspecified. Great, I thought - I can make her female. But it was a bit trickier than that, because if I had my protagonist notice that the servant was female, that might make it seem like having a female servant was somehow unusual - it would make that fact stick out in the narrative if I approached it directly like that. So I decided to use description, and show the hair or clothes of the servant in a female style. But I still had to make sure that was as relevant as possible. So I finally decided to bring in two other story criteria to help me: I needed to show that the servant caste is distinguished by tattoos on their foreheads, and also that my protagonist and his friends are afraid of these bodyguards. The final result was this sentence:
The servant's hair was pulled back in a bun so the curving caste tattoo on her forehead showed clear as a warning.
And it's the warning aspect that gets carried forward into the boys' next actions and responses, allowing both the servant's gender and her tattoo to be backgrounded.
The last piece, one I had more trouble with, was a description of setting. The setting of my novel is a very unusual one that doesn't fit with people's usual expectations, so I have to make sure to defeat people's usual expectations as soon as possible. Fortunately, there is an outside scene in Chapter 1 which I can use to establish some basic parameters (such as the fact that the entire city is underground). But in Chapter 1, it's nighttime, and the scene is set in the gardens of the Eminence's Residence, which is a pretty unusual place in that it has dirt and plants. So when I get to Chapter 2 and my second protagonist is running between buildings, I've got a quandary.
I don't want people to think that dirt and plants are normal and that everyone will encounter them if they go outside (because that's true only in the Eminence's gardens). On the other hand, running between buildings isn't a place where anything important happens, and the courtyard of the Service Academy isn't a location that will become critical later. My relevance support structures are few. So for now, I'm going to keep the description relatively short:
...headed out into the courtyard that separated the dormitories from the main Academy building - a single sheet of limestone worn smooth by centuries of running feet.
At this point I'm drafting, so who knows? I may come back to this location later and decide I need to change it because it needs more. But I will be careful, because at the moment I don't have enough relevance support to add much more than this, and if I need to add description later, I'll be trying hard to add relevance support too.
I'm going to keep thinking about it.