depends on who your working for, and depends on what kind of quality your client/employer needs. Deadlines can range from a week to 30 minutes.
If you're already working in the industry, you're obviously fast enough.
I concur with Coinpurse. Times vary a lot from project to project. Two extremes: At my dayjob I just did a bunch of sketches of different environments for our MMO to serve as loose ideas/color keys to make sure each area was distinct. They took about 40 minutes each. On the other end of the scale, I've been working on this huge set of underwater ruins for a short film that are much tighter and have taken a few weeks.
Generally, MMOs will need stuff faster than single player games, which will need stuff faster than stuff for the screen. If something is being seen by non-art people as part of the decision making process it'll often need to be more polished and take a little longer because of that. Likewise you'll want more polished concepts if there's time and the concept artwork might wind up as part of of the promotional material as well.
Work strategies and deadlines are another thing that can vary wildly from project to project. Most of the time the sort of stuff I personally do tends to be very iterative with fast turnarounds. Do a sketch in an evening and email it off to the guy I'm working with who'll usually reply back by morning. It's funny, but the smaller jobs are often the ones that are the biggest pain. Heh, you probably know the type.