I presume that ship maintenance takes a certain amount of time to complete if you choose not to spend money, just like any other game with this mechanic. That must be why it says "pay credits to skip maintenance."
For some reason though my fleet doesn't receive maintenance after a certain period of time. It has been in maintenance for two days now. There must be something wrong here, it seems like a bug of some sort. Or are we really forced to spend credits for maintenance?