If you're shooting around in the woods, a case should last you all day. If you're playing on an x-ball or NPPL field, a case should last you a good 3-5 hours depending on how much you shoot.
P.S. - are you using ramping or semi auto? If you're using ramping, you're probably going to get closer to 3 hours, if you're using semi auto, you're probably going to get closer to 5.