In a situation where you can take "any amount" of fatigue, there is no limit to how much fatigue you can take. The rules tell you what to do if you can't take a light fatigue or overfatigue, and also what to do if you'd take fatigue when your cooldown is full. Outside of those consequences, there is no limit.
This is particularly of note for the Palefrost Elk pet item which, when you would take any amount of fatigue, allows you to ignore the fatigue and put an HP chip on the pet instead. In combination with a situation where you can take any amount of fatigue, you can declare you are taking as much fatigue as you would like, and then ignore all of that fatigue.