1UP report quotes Sony head of Worldwide Studios Shuhei Yoshida saying that the NGP handheld will have "about the same [battery life] as the original PSP." The PSP's battery life varies considerably from around 3 hours if the UMD disc drive was whirring away, to much longer if played off flash storage. Sony's official estimate for average PSP operation time is 3-5 hours.
Meanwhile, Eurogamer has Sony Europe honcho Andrew House saying the device will have a "good, solid battery life because of two factors." The dual delights of life extension are apparently the card-based media (instead of UMD, which requires moving parts) and the OLED screen. Those potential power savers are joined by energy-sapping features like a four-core CPU, 3G, and other factors, helping explain the PSP-like battery life.