I've yet to exhaust an 8.4v 3800mah large battery in a single day in my M4, but I do have an 8.4v 1400mah as a backup that can be swap out from my secondary/backup MP5.
The last game I played at the new flag raiders was months ago. I did lay down a ton of suppressing fire with the M4 and that battery is still reading a little over 8.4v. Has yet to be charged. Been using that battery for plinking and extensive testing in both the M4 and MP5.
I am looking to get a 9.6v custom large pack made up for the new stock I'm getting and will probably have a 9.6v mini made for a compact backup since the 8.4's will no longer fit.