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.