That's very interesting indeed.

I get one reward per pomodoro, unless I chain the pomodoros together, in which case the reward matches the number of pomodoros completed (so if I do three in a row, 75 minutes of work, then I get 3 M&Ms). If I want to take a break, then I accept that I'll only get 1 M&M, instead of 2 or 3, after the next pomodoro.

In practice, then, I'm using variable intervals. Based on your feedback, I'll experiment with eating all the rewards at the end of the time interval, instead of devouring them after each pom.