Hi Chris, firstly, are you are charging it correctly and up to its full capacity? If so then you need to measure the current you are draining from it and divide the battery capacity by that figure, this should give you the run time you should expect.
For example, if you are drawing a constant 1Amp from the battery, you should see something approaching 4hrs of run time and if drawing 2Amps, expect something like 2hrs run time and so on. Any run time less than these figures would indicate that the battery is perhaps going west as they don't like being left flat for prolonged periods... this causes an effect known as 'sulphation' which is the kiss of death to lead acid cells. Replacements are now very cheap so it's not the end of the world if it has keeled over.
Regards....Chris