The general concensus of the last thread on this issue was that fatigue takes roughly 400 gathers to hit with a continuous recovery of ~30 gathers per hour. Due to that continuous recovery of gathers, the length of time it takes to hit fatigue varies with the speed of your gathering. If you do some hardcore continuous gathering, you can hit fatigue in roughly 3 hours. (See the thread I linked for more detail than you ever wanted to know)