In a tourist area - anything you’re foolish enough to give as almost everyone over-tips. Outside that narrow range (and I’m quoting Thais with whom I might go to non-erotic massage) 100 baht if you’re the foreigner paying. Thais themselves will tip less than that. I had coffee yesterday with a middle-aged Thai who had just returned from Pattaya where he had had a massage. He complained that the boy kept touching him up and he had keep telling the boy off. I think he tipped him 50 baht