Laptop batteries come in various capacities, including 6, 8, 9, or 12 cell models. Any of these should supply plenty of power for a wide range of tasks. Depending on the laptop brand, 6 cell models offer up to 3-4 hours of battery life. 8 and 9 cell up to 5 hours. Basically, more cells equal longer battery life.
For longer uptime only run programs you need, turn down screen brightness, avoid playing cds/dvds, and turn off wifi and bluetooth when not in use.
