How to find out power capacity of an automotive lead acid battery?
The easiest way is simply to ask the manufacturer. Battery capacities are measured in amp hours (Ah) and Exide quote 90 Ah for one of theirs, Century quote 90 Ah for one and 85 Ah for another and Challenge quote 90 Ah for theirs so yours is likely to be similar - find out the manufacturer and get the data sheet for full details of capacity and how it's affected by discharge rate, temperature etc.

In simple terms, 90 Ah means the energy it stores is equivalent to that delivered by a current of 90 amps flowing for one hour. Or one amp for 90 hours. But in practice, the voltage will drop as the battery becomes discharged so it's never that simple, and if you regularly discharge the battery by about 50% its life will be drastically shortened unless it's a deep-cycle type that is designed to cope.

Your 25% design goal is far more realistic, and would give you 1 amp for 22.5 hours. You'll need to know the actual consumption of your camera to get an accurate figure.