The Life of My Cell Phone Battery

I am in the process of charging my cell phone battery. It's taking a long time. When I first got the phone, charging the battery only took about an hour or two. My dad, since then, suggested that I keep my phone on for six to eight hours whenever possible, just in case someone is trying to get in touch with me. Lately, on days I don't have school, I've kept my phone on for pretty much an entire day.
Now that I've had my phone for about two years, I am no longer able to just charge the battery for an hour or two. Keeping my phone on for several hours causes the battery to run completely down. Charging for only an hour or two doesn't give me a full battery, so I end up charging the battery for. . . . sometimes four hours.