Back in the 1980s when you couldn't get good yeast, I often used yeast cultured from a bottle of Worthington White Shield for more than 3 generations without any problems. (It got expensive having to travel from California to England and back just to get a suitable yeast.)
Nowadays, I always stop after 3 generations at most. I'm sure I could get more out of them most of the time, but I have noticed that the yeast does change a bit from one generation to the next, and I've had a few batches with older yeast where the characteristics have changed very noticeably. As I can get up to 20 brews out of one vial of yeast, the cost of yeast becomes pretty insignificant.
-a.