I think he plays for a long time, well into his 50s if he can. Unless something dramatic changes with his demeanor (getting married and having kids hasn't seemed to change anything), then he'll play for a long time. He's way too competitive and way too dedicated to his sport. It's somewhat like Michael Jordan. Even when he thought he could turn it off and retire, he couldn't, coming back twice after retirement.
It's impossible to predict the future, but I just don't see Tiger stopping anytime in the near future. And honestly I expect him to win roughly 25 majors before he's finished. Also it wouldn't shock me to see him pull off a Masters or something crazy in his 50s.