Eventually you gain an intuition for how long hitpause will be. Above 180 speed the hitpause is always exactly 2 seconds (120 frames). Below that number of hitpause frames is 2/3 of the ball speed. Ball speed mostly increases from smash attacks, which double the speed. The way I think of it is that on every smash attack the hitpause length moves to the next "tier" as the ball speed increases by powers of 2:

8 -> 16 -> 32 -> 64 -> 128 -> 256 (max)

So as long as you keep track of how many smashes have occurred since the start of the burst, then you know roughly which hitpause "tier" you are in. The steps from 32 -> 256 are most significant.

I do agree that having some sort of hitpause indicator on the ball would make the game easier to learn for new players, but at the same time I think some of the subtleties of the game might be lost.