How long can a battery set between charges?
Basically all batteries start to discharge the instant they are pulled off the charger. Battery chemistry, battery condition, temperature, etc will determine how rapidly batteries discharge. I pretty much agree with Shortman, with a decent NiCd, three days off the charger I'd guess the battery should still have about 90% capacity or so. However, I'd like to point out that you should always check your batteries with a loaded tester before EVERY flight to determine if it is safe to fly... even if you just peak charged the battery ten minutes ago, check it. It only takes a few seconds to check batteries and it should always be a part of the preflight before each and every flight.
Cheers