I always rented more or less 1.5G-2.5G roughly 0.003-0.005 3h-4h hours. You can also rent some rigs for a day or even a month.
But I think it´s quite risky You have to pay beforehand (~1 BTC for a month or more depends on the GHs). But if the rig has a problem
you have to wait the owner sees it and resolves it, so you loose time and a lot of money is stuck - even if you get a refund. So better
mine just some hours. Before the dificulty change I gained like 400 to 600 in 4hours.