I would say that, a moderator would know when to step down in two cases:
When the reasons and purposes for him being a moderator have disappeared
When life has gotten to him/her
In my second point, what I mean by life has gotten to him is that he has absolutely no time to contribute. In your case, you have time but not as much, so I suggest you evaluate your time and if you want to step down that is your choice, if you don't want to then you can confront your moderator team (if one exists), or you can contact those above the moderators.
When you become a moderator, it should also be considerate that you have a life, meaning you may not be always available to fully contribute (unless being a moderator is an actual earning job), and so when you don't have the time then be honest with your self, evaluate the time, then tell the moderator team and those above it (admins) that you won't be able to fully contribute. This is very important, and so is communication in general.
That is all if you temporarily won't be able to contribute, if as situation is permanent then it may be best to step down and another take your place so disorder doesn't happen.