What is the exact cooldown for a defensive fast move?

I believe it randomly picks a number between 1.5 and 2 each time so the attacking rhythm isn't repetitive and predictable.

It is random. Minimum 1.5 sec plus an additional random amount of delay for a total of up to 2.5 seconds.

I think I read somewhere someone was saying it is randomized and fits a bell curve between 1.5s and 2.5s. (A lot of uncertainty in that sentence, so don't take this as a solid source.) Whatever the randomization method though, it averages to 2s, so that is the amount of additional time used to approximate defensive damage per second.