What are fast and slow decay timings in the Allegro A3201 low voltage meter?
Scanning the datasheet, it appears that fast decay or slow decay are achieved by leaving the outputs off (high impedance) or on (low impedance) but at the same logic level.

So the rise and fall times are determined by the driver output impedances (which are given in the datasheet) and the inductance of your motor's windings (which the chip manufacturer obviously doesn't know). So they cannot give a reliable answer in the datasheet.

I think you have to do the rest of the work yourself; either using the motor's datasheet or your own measurements of its parameters.