Yes they do last a certain time.
It's far better to top them up every day than to run them well down before charging.
If you take 2 identical deep cycle leisure batteries and run one down to 50% discharged and the other to 80% discharged the 50% discharged one will last approx 3 times as long as the 80% discharged one.
Got this info from a leisure battery manufacturers tech dept.
Cheers
Andrew