That's a difficult one to answer. Each studio has its own set of deadlines and expectations, which will also vary wildly by medium (games vs. television shows vs. advertising vs. films, etc). Generally, as you move into larger studios, the deadlines tend to become more realistic (read: a little more spaced out). The larger studios can charge clients more money, which the clients are willing to pay because they know the trade-off is a higher standard of quality. The studio knows that more quality will need more time, so deadlines become a little less intense once you get to a top-tier animation studio, generally speaking.
However, even the larger studios are increasing their productivity all the time (via new technologies, faster machines, better artists, etc), and the "average time to do a shot" is getting less all the time, it seems.
Of course, the biggest X factor in all of this has to do with the content of the shot. I've done a shot in a couple hours (of a hand, in the movie A.I.) but I also got bogged down for about 6 weeks on a shot in Hulk. On Transformers, we had a really streamlined situation with a really fast feedback loop, a lot of amazing animation tools at our disposal, and terrific animation rigs, all of which helped us get our animation time down dramatically. One shot I did of Bonecrusher on the highway only took a few days, while another took closer to 4 weeks - so, yet again, it all depends on the number of characters and what they are doing in the shot. It's always going to take longer to animate a giant robot tackling another robot in slow motion than it will to animate one robot skating down a highway. (There's an example I never would have thought that I'd have at my disposal! ha ha ha)