You'll find all the info you need here :
Climb Mt. Fuji
It'll take 5 to 6 hours from the 5th station to the summit.
A word of warning however. The mountain gets
very crowded during the peak season of July and August. As you near the top, it'll start to resemble a waiting line at a bank as you move upwards at a snails pace.
Personally, my plan is to climb it this November. I'd rather climb up from the base and carry a tent, sleeping bag, etc. than face the summer crowds.