Of course, it all depends upon what the warmup scene is like. If it's a taper meet for me and the 500, I'll typically do something like: 400 IM - easy kick-drill by 25s (too me, this is the best way to just ease into the pool, get the full range of motion for legs, arms, core) Then I might do something like 12 x 50 where I go 4 descend freestyle on an interval that would give me about 15 seconds rest above my target race pace (e.g., 45 seconds), trying to hit the last one at target race pace 4 on 1:00, going IM order, build swim 25, then build kick 25 4 free on that same interval, holding target race pace
At this point, if the race pace is easy and I feel warmed up, I'll do a 50 to 100 cool-down If I feel like I still need some more revving up, I'd do something like 1 x 50: easy - just to relax from the previous set of harder 50s 4 x 25: either doing cycle 4 - build-up, build-down (e.g., start fast and get easy), all easy, all fast
1 x 50: easy 2 x 50: trying to hit the race pace again 1 x 50: easy
In general, for a taper meet, I'm going 1,000 to 1,250 warmup. If it is a mid-season meet, it takes me a lot longer to warm up, so I'd go 1,500 to 2,000.
The other thing I like to do, especially for my 400 IM and 500 or longer, is to time my warmup so that I finish 20 minutes ahead of the race. That allows me enough time to dry off, put on a fast suit and get behind the blocks, feeling rested, but still warm.