Really depends on how much charge they are loosing. I personally don't tend to get through more than a few hundred rounds in a few hours so I can go a day on a lot less than that (lipo). But really depends how much of the 3300mah charge remains available and how much you shoot. The rule I have heard is about 1 mah per shot so 6600 mah worth of batteries for a day is quite a lot more than most people I see using. Only you know can estimate how much charge you think remains in those batteries and how much you shoot on a day.
Right ok thank you can always tone it back a bit I'm estimating over half and its a more powerful battery apparantly than the gun requires an m4 so 2 batteries in theory is making a whole
Haven't really used them before so I have no real clue what there capable of
NihM and NiCad batteries don't hold charge well if you leave them hanging around for weeks, but if you charge them fully a day or so before you intend to play then they should hold charge enough to allow you a whole days play with them. Lipo batteries are much better at holding charge when left disconnected. One of mine only lost about 10% over 5 months.
If you have the right charger do a cycle charge on it although nihm batteries don't need to be discharged a cycle may help to bring the hold back. Also try a slower charge rate charge the battery on a lower Amp setting.