I would say as long as you want to and as long as you can. If you play ten minutes and your fingers are hurting or your hand is cramping, then it's too long.

Since you're first getting started, your finger tips will get sore pretty quickly. It will take a few weeks for them to toughen up so that you don't notice it much. If you're like me, your left hand will also not be very flexible or strong, and it will tire quickly. Both of these things will probably cut your effective practice time while you're getting started. I've been playing for just over two years now and can practice a couple hours without really being limited by hand strength or finger tenderness.

I think most people will also say that more frequent practices of shorter length are more effective than occasional very long ones, all else being equal.