You can go to 20-25g of creatine mono. There's no problem with kidney function or anything else at that dose. Drink plenty of water, but that should be a given anyway. I would not recommend that dose on an everyday basis long term though because it's not cost effective.
Basically the effectiveness of CM will decline as your body sort of burns out on it. 20g a day long term ends up just getting wasted. I find cycling CM 3 days on/ 3 days off is most effective. I take around 20g in 3 doses per day. 5g pre-w/o, 5g post-w/o, and 10g in the evening. Then 3 days with no creatine. It's basically loading for three days, then giving your receptors a rest. I have consistent strength gains at all times with this method.
When I took around 10g as the "maintenance dose" as recommended on most labels, I found that the effectiveness lagged after a few weeks. You can search the AST Sports Science website for more details on creatine cycling and the science behind the method.
Good luck.