Good question for which I don't have a definitive answer!
Have had my trusty Seiko RN/RAF watch for nearly 16 years. Along the way I've then then needed a waterproof watch, then needed a dress watch, then the internet happened and then... you know the rest :D
I'm still a relative noob around here and still think a chap only really needs 3 watches max. I seem to have failed on that count already :D