A typical, fully charged, standard capacity battery should last at least 2 to 3 hours under a normal load, if not longer, with high capacity batteries ranging somewhere in the 4 - 5 hour neighborhood. There are numerous factors that can shorten the useful charge of a battery:
1. Excessive screen brightness -- turn it down to a mid-range setting
2. Running multiple, power-hungry tasks -- avoid certain activiites until you can plug your laptop into a wall socket, ie, avoid the temptation to play a CD or watch a DVD with the volume cranked up, while wirelessly downloading the latest release of your distro, while also backing up your personal data to a USB stick.
3. Battery age, or lack of battery "exercise" -- don't be afraid to let your battery drain all the way down to the point where you start seeing low battery warnings, but then let it fully charge up overnight. If you are using a Li-ion battery
this is not as much of an issue as it is with other batteries, but over time Li-ion batteries gradually but steadily lose their ability to hold a charge, regardless.
4. Higher hard drive RPM's = shorter battery life. It's just simple physics that it takes more energy to spin a disk platter at 7200 rpm than 5400 rpm, so if you happen to be using a faster drive, the improved speed comes at the cost of shorter battery charges. As economists always say, there's no such thing as a free lunch
If your battery is less than a year old though, and you are getting terrible performance, it may just be defective, and it may still be under warranty. If I were you I'd check that out possibility before shelling out for a new battery. Lastly, if you do buy a new battery, buy it from IBM (errr, umm, Lenovo now) rather than to go with a no-name or off-brand manufacturer. Good luck with it. FWIW, I own a T40, and typically get about 2.5 - 3 hours from the standard (6 cell) battery, and approx 4.5 hours with the high capacity (9 cell) battery.