AH is the capacity of the battery in Ampere Hours. In theory, you can use it for 100hours using a 1 amp bulb, 25 amps for 4 hours or 100 amps for 1 hour.
Divide the capacity by the amps and that's yer lot.
------------- Some days you are the dog,
some days you are the tree.
Tanya Batteries are an excellent online retailer of batteries, including a good range of leisure batteries.
If you're planning on being off EHU much, I'd suggest getting a larger capacity battery but this can be influenced too in part, on your usual use (lighting, TV etc).
Don't forget that it isn't just a case of buying a battery and fitting it. Your cables might dictate which way round the terminals need to be, plus some battery boxes might restrict the physical size of battery you can fit.