Well, it is mentioned in the film that 10 hrs under heavy sedation = 1 week in level 1 dream, 6 months in level 2 dream, 10 years in level 3 dream That's the original Inception plan.
So the fourth dream (Limbo) would last for say, 200 years (that's if each level multiplies the period by 20, roughly), but again Yusuf said he doesn't know for sure.
However, it seems that not even a day has passed in level 1 dream throughout the whole mission. This is due to the unforeseen circumstances such as Fischer's subconscious being trained (heavily armed and capable of finding the intruders quickly) and Saito is hurt and dying, so their mission needs to be completed as quickly as possible. So Saito supposedly lived in his fifth dream for less than 28 years theoritically, which is still too much! :)