No idea what Spoon is trying to say in his answer. :S
Ok when looking at batteries there are two parts to notice the "xxx"mAH and the xx.xV
The mAH will equal to how long the battery will holds it charge under normal conditions. While the V for this purpose will control how fast your rate of fire will be. So a 7.2v battery will have a rather slow ROF under fully auto than a 9.6v battery will.
You also have two types of battery that are common in airsoft. Lipo and NiMh(There are other types, but these two I think are most common). For most cases which one you chose is personal preference. But I always go for NiMh batteries because they are less of a hassle than Lipos (whole other story)
Now the main issue with batteries is that they perform worse in cold weather than in warm. so Its always a good idea to have a spare just in case any way. But to your original question, If you play stingily, you can make that one battery last a whole day. But if you like to go full rate of fire non stop with high caps. Than you may want to have a few spare to hand.
(Although I always keep about 3-4 batteries in the car in case some one needs a spare.)
Any way, Hope this helps.