He should use it for only as long as it's comfortable. Doing it for longer will probably just make him hurt more which isn't good. It's like doing too much, it's not a good thing and pain is not gain. He should find the lowest setting that is comfortable and maybe over time try to go up by one number and see if it's good or it hurts.

I had a tens myself a couple of years ago that I used for one day. It was way too painful for me even on the lowest setting.

Karen