I've got a 12", 1,33Mhz that's about a year old. I use it in university, and with dimmed screen it still gives me about 5 hours of text-editing. (Thats without Wi-Fi though)
With wifi and bluetooth turned off and teh screen set to about medium brightness.. i get like 4.5 hrs of battery life (brand new pb.. one week old now) and with wifi on i get about 4... so yah!