It would be difficult to tell how quickly you will kill the LEDs, but I am willing to bet we aren't talking the hundreds of thousands of hours an LED would normally last for, depending to what extent they are over driven it could be minutes or hundreds of hours, either way I am not testing it. Programs on ovi store should really carry this warning.

I am sure someone will eventually offer a program to do this, but my 2p worth is that you should avoid using them as a torch, otherwise nokia would have already offered it as an option or atleast allowed a light on mode for video (which is a probably a use case they have considered).