What do you mean by diminishing returns?

15 minutes twice a week gets you a certain amount of results

You will not get twice the results for 30×2 or 15×4.

2 hours three times a week doesn't get you 4 times as much as 30 minutes three times a week

The amount of results you get diminishes significantly for time and effort put in.

Having a cut physique like an action movie hero is nearly a full time job but being basically fit with the health benefits that come along with that is actually a pretty small chore if you do things right.