Since distance/relativity stop and quantum mechanics take over at Planck's length, actions across lengths less than this boundary are meaningless.

The fastest speed attainable is the speed of light (apparently).

So, if we take the smallest length and divide it by the fastest speed, we get the time it takes for the fastest thing to travel the shortest distance. Times shorter than this amount simply do not make sense.

SO:

Planck's length is about 1 x 10^-34 m
Speed of light is about 3 x 10⁸ m/sec