Lately my Samsung Galaxy Ace phone dies quickly even after being full charged. Yesterday it only lasted for 20 minutes before telling me to plug in the charger or shut it off. I had it fully charged before I even turned it on.
It sounds like your battery in your phone has pretty much reached the end of its life if it is only lasting 20 minutes. If the phone still has warranty you can bring it in for warranty servicing, if not you can purchase a new battery but sometimes it's more beneficial to upgrade to something newer than spending more money on an older phone.