Well, I think that's not a hard and fast rule, they keep growing and blooming as long as they stay healthy. There's a picture somewhere on this forum of one in Golden Gate Park in SF that is probably 20 years old and 30ft tall. Still going strong. It really depends where you are and how much TLC you can give them.
Here in Florida, it's just really difficult to keep them healthy for any length of time especially growing in pots. As mkb said, much easier to have a nice healthy plant if you start new every 5 years or so. It's not like they're hard to start, or grow slowly. I've had a cutting from Monster White go from 8in. tall in spring to a 4ft x 4ft blooming bush by fall. I like them to be bushes and bloom sooner, so I take cuttings above the Y.
