How long can an un-used charged extra battery go un-used for without needing to be recharged?

I have two extras I rotate with the original. The longest I know for sure I went was 2 weeks on one battery and it was still at 100% before I tried to charge it.

Thanks for the benchmark type test.

As we continue to build off that:

This extra battery (at least) sat on a shelf for about 2 months, popped it into the phone to see the meter and it was still on full. Popped it back out and put it back on the shelf.

Impressed with this Blackberry thing (life, recharge speed, etc.)

Of course that is good, and will/may vary from battery to battery... and according to the temperature of the storage area, may deplete faster in a hot (100*F degree+) climate.

Standby is rated at 14.4 days. But as JSanders says, a lot depends on the environment where it is kept.