depends on the processor, the crusoe and centrino laptops get way better battery performance than say a Pentium 4 would. The centrino laptops get great battery life. I got close to 2 hours anf 45 minutes when testing my mine doing 24/96 recording. Some of the trick is proper power management. When you have it open, run the LCD brightness as low as it can go and keep the laptop closed as much as possible since its the LCD that drains the most power. 2.75 hours was what I got with the laptop closed only checking things a few times. Also take the DVD or cd drive out, that also draws power, even when not being used.
I figured witih my 2nd modular bay battery I can get close to 5.5 hours with the 2 internal laptop bats. More than enough for a show. Also need to take good care of your battery. When running AC power, keep it out of the laptop, and inbetween uses, keep it about 1/2 charged and store it somewhere cool.