How long can you work on making a routine task more efficient before you're spending more time than you save?
Of course, he's not the first person to ask this question, but the visual is handy. Note that the times are calculated assuming you'll save the time over five years.
For example, I've been pondering how to shorten my showers. If I can shave off 1 minute daily, I should be willing to invest up to (but no more than) an entire day to do it. If I think I can shave off five minutes preparing my breakfast, I should be willing to spend up to six days attempting to do so.