Like Rentahamster said, it will reset exactly one week from the very first time you started gaining SP on that class. So if the first time in the game you kill a mob with an axe is 12:00 p.m. on a Wednesday, then your fatigue for Marauder will reset every Wednesday at 12:00 p.m. If the first time you equip a needle and do some weaving is Friday at 10:00 a.m., then your Weaver fatigue will reset every Friday at 10:00 a.m.

Here's a video on the fatigue system (link in text).