Fatigue isn't something I predict based on my schedule. It's something that happens over time, I've called in a few times with it. How do I know it's time?

Am I laughing at things that aren't funny? Unable to think through problems? Getting easily ****ed off? Do I feel that I couldn't pass a checkride in that condition? For me those are all signs. If all hell broke loose and I don't know for sure that I'd be reacting properly, then it's time to call.

Ask yourself if you'd want your kid/loved one in the back of a plane flown by someone in your condition. If there are any reservations then you have no business flying.