a 9.6v 3600mah battery would start you out around 1400rpm when it's topped up, and then slow down to 1200rpm
You can avoid that by not topping up the battery, and it'll last atleast 6000rounds
The 2 most common battery types are 8.4v large, and 9.6v mini. They give you about the same rate of fire, and it's 900-1100rpm. There's a few variables that affect ROF other than your battery; gear ratio, spring, type of motor, condition of motor, quality and gage of wiring in your gun, whether or not you have a mosfet switch
Not all batteries are the same, you can't just slap any type of battery in your gun and go, you have to know HOW TO USE those batteries safely.
NiCd, NiMH you can beat the crap out of them and they work great. They're noob proof.
LiPo's can spew burning hot gas in your face if your expose it to over current draw, or damage it then charge it up after.
You don't seem to understand how LiPO's work, so read this;
http://www.airsoftcanada.com/showthread.php?t=99899