My limit to playing on the Vita is about an hour cause it's not super comfortable...s
I find the vita to be fairly uncomfortable to hold.
Sorry that this is a little off PSN deals topic, but if you find the Vita uncomfortable to hold, try investing in one of these. I got one years ago when I was having problems playing Uncharted: GA for more than 30 mins and it completely changed how and for how long I can use my Vita.