I have climbed Mt. Fuji and from the 5th station it will take about 6 hours depending on how many times you stop to rest. Going back down should only take about 5 to 5 and a half hours. It is very hot at the bottom and very cold at the top so you must be sure to pack very warm clothing, especially that which will protect you from the fierce wind at the top. I did not do this and was shaking pretty violently the entire time upon reaching the summit. I also recommend a night climb so you can be there for the sunrise so long as the clouds stay away.
A walking stick is also invaluable for your ascent and they can be bought at the 5th station. I would have died without mine. It also is a great souvenir since at each rest stop up to the top you can brand the name of the station on the stick and how many meters you have climbed. They charge money for it so be sure to have plenty of cash on hand. The drinks are also very expensive on the mountain so be sure to pack plenty of drinking water. There is actually a vending machine with hot coffee at the top although one small can costs about 5 dollars. I did take pictures but have been too lazy to put the on the website as of yet.
A good night and days rest is also advisable before the climb. I saw many poor souls who thought they could spend time drinking and then climb the mountain only to peeter out extremely early. If you do the night climb be sure to have head lamps for everyone.