Calculating how long a battery will last

[Translucent Daydream]

I am going to make a six volt lighting system for a custom flag pole that is going to run off of lantern 6v batteries. (like this http://rocky.digikey.com/weblib/Ener...hotos/1209.jpg)

I am trying to determine how to get the bigger bang for my buck, so I would need to calculate how long different variations of lighting setups, neon, LED, etc, would last. I am pretty sure you could do it if you could calculate the total draw from the lights vs the specs on the battery. Is there a decent way of doing this mathematically or otherwise?