Question isn't what is reactable necessarily (although this does have something to do with it), But what is what is fuzzy blockable.
I would say about 2-3 frames is the absolute minimum and that is tough. 5-6 frame differences is much easier.
Say a character has a 10 frame low and a 14 frame overhead, your general rule of thumb would be (say after a reset) to block low then high no matter what happens. You know the low comes out faster so you cover that option first, the overhead is slower so that is secondary. So you spend about the first few frames, blocking low then switch to high, since by that point the low is less likely to come out than the overhead.
