Battery run time is going to depend on your style of play. If you fire semi auto or in short burts using standard mags or a single hicap, you could stretch a low mah battery through out a day. If you plan to go in spraying away, plan to have to recharge or change batteries. I have made it through a day on a single 8.4v 1500 mah battery during milsim events. On more casual days, I have burned up a 9.6v 2000 mah battery in half a day. Your charger could also effect your run time. Cheaper chargers tend to give you less peak than more expensive ones.
To sum it up, there are many things that are going to affect your run time like temperature, semi auto or full auto, charger, and bunch of other factors. You are going to have to feel it out yourself. If your are conservative and with a good charge, I don't see why you couldn't make any battery last all day.
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