The best way to calculate the battery life is in terms of photos. It can take a minimum of 50k photos on a single charge, so 50k photos * 30 minutes == 2.8 years. The battery self-discharge will deplete before that though, so I'd expect 12-24 months.

Note though that the SD card can only fit 20k photos, so the battery typically lasts longer than the SD card, and after that the oldest photos are overwritten!