I've heard the Archean-Proterozoic boundary is one possibility for the beginning of the flood. I've also heard the PreC-Cambrian boundary.
The end of the flood appears to be more contentious among the YEC researchers. Some have proposed the beginning of the Cambrian, the Carboniferous, the Permian, the Cretaceous, and the Quaternary.
Basically, they don't have a clue. When they're willing to pin themselves down to a time period, they get in trouble because they can't explain all the details.
Hi, Rox,
Don't forget the recolonization theory. You know, the one where it's all over by the end of the Archean. This one has the incredibly convienient feature that all evidence of previous life and civilizations was comletely destroyed by a massively destructive event in the Archean. Life then began recolonizing the earth in the Proterozoic. This all makes a lot of sense and many find it intellectually satisfying. After all, an event that would metamorphose most of the existing crust would certainly have destroyed any evidence of its passing.