When I go to a festival I usually stay at hotels, so I bring about 30hrs of batt life and charge over night. And If I don't then I'd bring enough for the entire time. But that could be a lot of money in batteries if you have an inefficient rig. Mines a little box and m10 so the m10 I change batteries once at the beginning of the day. The little box can run 10+ he's on a single Rc bat so its very easy.
Also some festivals supply power in the tapers section if there is one. But I wouldn't anticipate that, its not very common