The problem with ALL rechargeable batteries is that they "REMEMBER". Good battery maintanence to vital. If you buy a new battery you MUST fully charge it from day 1 with NO LOAD. This means you need to put the new battery into the laptop and plug in the mains charger WITHOUT switching you laptop on. Leave till fully charged then power up the laptop am leave powered up till the battery runs out TOTALLY. Then FULLY recharge battery as mentioned in the first part of this.
After all this you should be able to get some decent time out of your laptop.
As mentioned by other users, battery life depends VERY much on what you are doing with your machine when in use.
Mike