I don't think the LEDs will burn through the battery too much, but running them for too long may cause other problems--I'm not sure what, but speculate maybe burning out the LEDs themselves (?). I stumbled across this in the kernel source from kernel-2.6.28/arch/arm/mach-omap2/board-rx51-camera.c
Code:
/* Must be limited to 500 ms in RX-51 */
.max_flash_timeout = 500000, /* us */
/* Must be limited to 320 mA in RX-51 B3 and newer hardware */
.max_flash_intensity = 19,
/* Must be limited to 50 mA in RX-51 */
.max_torch_intensity = 1,
It appears to indicate that the flash can only go for 500 ms (half a second on the RX-51 (which is the N900). It doesn't say *why* it "Must be limited to 500 ms" though.
-Jeff
http://wiki.maemo.org/User:Jebba