If you don't like the longer barrel on the m16 but would like a full stock then get a full stock m4, like the CA M15A4 tactical carbine or something similar, that has the full stock of an m16 but the shorter carbine barrel of the m4. The benefits of the full stock is you can use a large battery rather then small (well you can use a large with a regular m4 but you'd have to use a peq box or a battery bag, both solutions I find awkward). Large batteries have a much longer run time then small batteries.
About the run time of a battery. That depends on three things, whether the battery is a large or a small, the quality of the battery (ie manufacturer) and the battery's mah. The mah is the most important of the three, followed by quality then size. I've got a 3300mah large type NiMH and I've gone over nine hours and easily over fifteen hundred rounds without coming close to draining it. On the flip side, some teammates with 1200mah small types have drained them in the span of four to six hours of general skirmishing.
As to your question of how long batteries last. If you get a good charger and take care of them they'll last you years. Always good to have a spare, just in case something happens to your primary battery, or one of your teammates forgets to charge their battery (which happens way more often then you'd think), but you don't need more then two (unless you go with low mah small types in which case three or four wouldn't go astray). I can't say anything about the cost of batteries in Asia, but I use cheapbatterypacks.com
Oh and +1 to Amos and notom66866, stock batteries suck and blowback is a nice novelty but are more trouble then they're worth for a serious skirmish gun.
Classic Army M15A4 Tactical Carbine
Classic Army M15A4 Tactical Carbine plus shorty CA M203, tac light and reflex sight