Have you seen this video from Dave from the EEVBlog: https://www.youtube.com/watch?v=Dh0xYu8YvaE&t=642s ? It explains the basics and how to measure current consumption on a board with a microcontroller. You can use the same approach for an RPi.
Note how Dave uses the integration function of an oscilloscope to accurately measure the power usage when the device is active (in your case that would be: when taking a photo).
To determine battery life Duty-cycle is a very important factor. To illustrate look at this example:
Let's say that your device uses 0.2 A (average current, you might have to use Dave's method to determine this accurately) when it is active.
When it is inactive (in sleep mode) it uses 100 uA
When the device is active (needs to take a photo) it is active for 3 seconds.
A photo needs to be taken every 5 minutes.
Now we know enough to calculate the average current consumption:
Per hour the device will be active 1 hour/5 minutes = 12 times
That times 3 seconds per photo = 36 seconds per hour in active more at 0.2 A
That's a 36 seconds / 3600 seconds (on hour) = 1 % Duty cycle.
So that's an average current of 1% * 0.2 A = 2 mA
Add to that the constant 100 uA of the sleep mode gives: 2.1 mA average current.
So on a 1000 mAh battery your device would then last:
1000 mAh / 2.1 mA = 476 hours which is equal to about 20 days.
Now look at the numbers and think what happens if you would:
take more photos, every 2.5 minutes instead of every 5 minutes:
the battery life would practically be halved
the sleep mode current would double:
not much would change.
This is just an example so depending on your actual numbers the situation might be different but I hope it is clear now how to do the estimation.