As with most anything, it's going to vary by person and situation.
Onset of olfactory fatigue for me can be as short as ten minutes.
Since the 'fading' of the fatigue is gradual, it's hard to say precisely how long it takes. I would say it is at least hours. You might want to try using a nasal spray and see if you can speed the process. As I understand it, the hairs in your nose get coated with the fragrance and re-radiate it.
I've noticed that I lose the stronger notes of a fragrance. (like the vanilla in Obsession) I haven't done a test, but I'm pretty sure my overall sense of smell is not as sharp once I've entered OF.
The general answer to the problem is not to apply as much fragrance near your head. (or on it!)
I switched my application method from pulse points on wrists and neck to on my chest under my undershirt for that reason. Where before, the fragrance on my neck would cause OF and the fragrance would evaporate off the warm skin of my pulse points in four hours or so, now on hair and some on the undershirt, it doesn't evaporate as quickly and if I want to smell it, I can tuck my nose in my shirt and pluck at my shirt for a bellows-type action. This will present the fragrance for up to twenty hours or so later. Of course, this method produces less sillage.
The other answer is to stay on pulse points but carry an atomiser to reapply. With that you have to be careful not to overapply (which is tricky when you're already in OF).
Hope this helps