You don't say where or for how long it is. I know many find this a vexed question. In some places the masseurs are on staff, in other places they get a proportion of the amount you pay at the desk. I usally go for the two-hour job at one of the traditional places in Bangkok (330 baht), and find they're exceedingly well please if I give them 150 baht tip; I've seen Thai customers give 100 baht, and I've only ever once, in having massages a couple of times a week for the past 15 years, ever been asked for or even seen a masseur hovering as if they expect a tip. 50 baht is, I'd say, barely acceptable for a two-hour job, but probably acceptable for one-hour (which is what 250 baht sounds like). At least you're not asking the usual stand-by - "How much tip for a boy" - and in most cases the masseur is working a damn sight harder and longer (although slightly less intimately)