There is a level when you hit Fatigue..

is like the first you hit Fatigue; that is 10% and gradually increase until you will not gain any SP/EXP anymore, which interms telling you to take a rest -or- play other classes.

While playing other classes that is not fatigue, the fatigue classes will gradually diminish depending on how far you've grind on the fatigue class. but heck, it'll reset by the end of the 7 day cycle. No biggie.