it's not per hours that you count a airsoft battery life. It's per shots. You should get about 1000-1400 shots. Enough for a small game but if it is a big one, get a backup battery or a 2000mah +.
A non-modded M4 should be able to take this : http://www.airsoftgi.com/product_inf...oducts_id=4151
because the battery is in the foregrip or if you hate always
opening the foregrip to recharge your battery ... you can get a batter box http://www.airsoftgi.com/product_inf...oducts_id=1883
. With it, you can have access to the battery easier and
you can a bigger battery at a lower cost. If you are getting a M120 Spring (Systema for example) You need to consider upgrading other stuffs like piston , piston heads and better gears because you put more stress in the internals by shooting stronger.
It's like a car ... you want to put turbo? You will need to upgrade to forged pistons and other stuffs to handle the turbo heat.