Some laptops with Pentium M's can last upto 5 hours. Mine was purchased as a desktop replacement, so it has all the power of a full desktop, in a mobile case.
All depends on the processor for average figures and of course this is then impacted by what your doing, with games and DVD's being the "usual" most intensive things on a battery.
Of course other factors have an impact on battery life too, including GPU (i have a Radeon Mobility 9700 - if it was not a GPU just onboard graphics, again the life would be better) and screen brightness.
Desktop processors: 1 - 2 hours. (eg. Pentium 4)
Desktop Mobile processors: 1.5 - 3 hours (eg. Pentium III M/Speedstep, Pentium 4 Mobile)
Mobile processors: 2 - 5 hours (eg. Pentium M, Core II)
btw, not trying to take anything away from the AMD's. just their not very good for battery life even with their mobile processors. Intel had the Pentium III to fall back on, low power and heat, AMD have had to develop a whole new technology.
Damn I am going off topic...
Back on topic now hehe