Bought a male and female Watanabei together from LFS. Tank is 4 x 2.5 x 2.5' and the other inhabitants were a small ocelaris clown and a chevron tang. The male chased the female like crazy at first, but this tapered off. After 2 or 3 years (can't remember), they would chase once a month or so, when I happened to be looking of course, and then the male suddenly was gone. Looked for jumped carcass, but there was none. He must have suddenly died somehow. In any case, the cleaner shrimp would have chowed him before I saw anything.
I guess my point is that they can stay male for several years in a 150. He was healthy looking. However, I have way fewer fish than most people have in a 150. I still have the female and the other fish. That was two years ago or so.
G1