The flash on my G2 (HTC Desire) just stopped working. I used the Torch app for a few minutes at a time maybe once or twice a week for the last year or two, always on low brightness. Three months ago I used it for about 30 minutes walking back to my car on a dark beach. The battery on the phone went from 90+% to dead in that time.
When it died I had turned on torch and noticed the LED was much dimmer than normal. After about 5 seconds of being on it brightened for a second or two, then went dark. I haven't gotten it to turn on since. I've rebooted, power cycled, tried both torch and regular camera flash. It seems to be dead. Obviously it didn't burn out from overuse at the time I was using it (for 5 seconds) and the previous times I'd used it in the last week or so were as a regular camera flash. I never used it for more than 5 minutes except for that one time on the beach and I don't think I ever had it on high brightness for more than 60 seconds.
So yes, I think there's a risk with using the flash as a flashlight even minimally like I did, but it probably varies greatly by phone and LED. In my experience, LEDs aren't actually very reliable. I had a 5-LED flashlight where one LED varied from being completely out to flickering annoyingly so I returned it and the same thing happened to the replacement within a few months. One of the tiny LED lights on a "microscope" USB camera I bought also has problems and won't turn on until you turn the brightness to full, then it goes out when the other 4 LEDs are turned down to 75% or less.
