Assuming the battery is 800maH (milliamp hours), and you're drawing 25mA from it, then (in theory) it should last 800/25 hours = 32 hours. However, that's in theory, in practice it can vary quite a lot depending on the actual current drawn, and the maH rating of the battery is usually done at the optimal loading, to make the battery look good!.
So only use the maH figure as an indication, but test to see what actually happens!.