Check the mAh rating on your battery and divide by mA to get hours.
So, if your 9V battery is rated at 600mAh and your device draws at a maximum of 25mA, then your battery would last 24hours in theory. Chances are your battery will not perform up to its specified rating, so I would drop 25% of that time. You can always do a dry run to confirm how long your rechargeables will last.
600mAh/25mA=24h
Someone correct me if I am wrong.