Hi Ben, that depends on what you want the descriptor to tell you.

If you want the descriptor to tell you the size of the hypertrophic effect, then you would just count the number of stimulating reps.

The number of stimulating reps would be the last 5 reps before failure in each set. If the set is not performed to failure, then the number of estimated reps before failure should be subtracted from 5 to get the number of stimulating reps. For example, if you stop 2 reps before failure, deduct 2 from 5 to get 3 stimulating reps for that set. We cannot use percentages of 1RM to calculate the hypertrophic effect, because the number of reps that are possible with a given percentage of 1RM varies between individuals too much (and also with fatigue).

If you want the descriptor to tell you how much muscle damage is caused (and therefore how long the recovery period should be), then that is quite a bit harder to measure. The recovery period will be slightly longer with light loads than with heavy loads, because of the accumulation of more metabolites, which leads to more muscle damage. Similarly, it will be longer when training closer to failure, than when training further from failure. And it will be longer with higher volume workouts (more sets) compared to lower volume workouts. But recovery time from any given workout varies a lot between individuals too. The best approach is to monitor repetition strength from one workout to the next, and if it is lower than expected in a subsequent workout, then recovery is incomplete, and training frequency is too high for that type of workout.