How to find it: do MD5 on some random input. Then do MD5 on that output, and repeat until it converges. The problem is that you could end up in a cycle, instead of at a fixed point.

The speed with which it can be found: I'm going to wave my hands and claim this is in "Analytic Combinatorics" by Flajolet and Sedgewick. Seriously, though, under the assumptions we've been throwing around here this is a "random mapping" and these are reasonably well-studied objects.