The mathematics of favors
As Chapel burns through at least one favor per show, and in several he used 2, plus in one show he did a freebie, and in another, he was paid, so no obligatory favor, he would run through his favor catalog very fast.
More realistically, it should have been the "helpee" would be obligated for life, with no limit to the number of favors, or at least a limit of $1 million dollars worth of favors. After all, Chapel was using these to help other victims like themselves.