At below level 10 it's easy. For the first 4 or so level no matter what class you are you'll level fast. Then as you get higher it changes a bit especially depending on your class.

I found that warrior and monk level fastest at around level 7-11 if you solo, while other classes need to party a bit earlier to get good exp.

At least that's the way I've noticed it.

Although as you get to higehr levels liek around the 20's and such it'll take you maybe about 3 or so hours if your ina good party in a not to populated area, leveling on some good monsters to be able to level. (Although that 3 hours was more so a low guess, usually it's longer most times, especially under not so great conditions)