"It was flat when I got it."
If it had been sitting discharged for very long there is a good chance it will not recharge, or your charger doesn't have the ability to properly charge it.
First, check it's static voltage .. if it is below 4v, a typical home charger won't charge a battery this low. It's a fail safe in the charger to keep you from harming it. If it's below 4v, take it to a battery shop that can recharge it (not a auto parts shop!)
If the voltage is up, it just needs much longer before it will charge. A fully discharged battery like that should take anywhere from 24-48 hours at 2amps - but it may take 12-24 hours before it begins to accept a charge.
So you're realisitically looking at a 24-84 hour charge cycle.
Leave it on a 2 amp rate until it starts to charge, once you see your hydrometer reading come up 25 points or so then you can switch to to a higher rate. Slower rate takes longer but will warm the plates up slower and reduce the chances of a dead cell.
Just keep an eye on it's temperature, if it's boiling or not, and keep checking it with your hydrometer. Use a swing-arm style hydrometer, the ball type are junk (really.)
The battery you have is probably 35-55ah. It's not a deep cycle battery, so like what was already said it won't handle being discharged and recharged as well. The best thing to do is to not let it discharge below 75% - this will greatly help keep it from doing bad due to cycling. You can get a lot of cycles out of a regular auto battery, but the chances of a cell not charging back up are greater if you cycle the battery below 75% discharge.
When this one goes bad, go buy a 12V 35AH AGM style battery. They are commonly used in wheel chairs and electric golf caddies - they can handle the cycling and best of all are completely sealed and easier to transport.