15 to 20 percent unless you are never ever going back and then still 20 percent. It's part still of the hair cutting bargain. I don't tip the people at my salon because it is more so a medical establishment at the high end but these people often get sub-minimal wages in the States. It's completely different in Europe and you only leave a pourboire for a bartender say, who treated you really well because they get paid salary.
I have only had my hair cut about 5 times as an adult. I refused to let anyone peer down on my crown from 19 onward and I am pretty good at it. The poor bald guys that come in for a haircut have to pay full freight and full tip for a tiny bit of fringe but some people do it for the interactive experience especially in some barber shops.