How long do you play for?

I've been playing micro limits for four or five months now, and have always had a problem staying at the table. I play at least six days a week, often multiple sessions per day. However, my sessions are usually very short.

One of two things happen when I play. If I start off in the first few rounds of blinds and am up a fair amount, usually 4-5BB I convince myself that the session was successful, and that I should end it on a positive note. On the other hand, if I go down 4-5BB quickly I start thinking that I'm not playing good, and I try to either get back to even or just leave.

Obviously this is a mental problem, and I'm convinced that I'm costing myself a lot of money in the long run by leaving so soon, especially when I'm up. So how long is a normal session for you guys? Would a good solution be to set a specific time frame for me to play in? Or could that become a negative? Thanks.