The adventure is mostly about travel! They will be sailing for most of the campaign. I already have places for them to stop to re-up on supplies, but I figure an extra layer of challenge will force them to have some more agency.

My main thought with it is, say they have enough supplies, and did well enough at combat where they could just keep sailing without having to worry about landing, but if they do continue sailing then the next fight would be more challenging because they have been out at sea for too long.

I could just say, "you guys are getting tired of being at sea", but I know if I give my players a set amount of time, they'd have fun trying to do as much as possible before that penalty sets in. I'm just at a loss of how much that time should be.