The easy answer is, yes you can. How long you can keep doing this depends mostly on how good your sanitation is. Once contamination gets a foothold, you're done rolling. Of, course, you could always acid wash the yeast to reduce the bacterial load after you re-use it a few times. A lot of folks don't like to go past a certain number of generations when re-using yeast, but that usually pertains to re-using it from a whole batch of beer. The smaller the amount of yeast you generate, the more times you can keep doing it. For example, if you plated yeast, you could re-plate it indefinitely because the yeast cells aren't multiplying that awful much before you grab a few cells and re-plate it. So a re-plating event isn't considered a generation. When you make a batch of beer, the cells multiply to a very large number in comparison and that's where you want to start keeping track of how many generations you've gone. Doing it with starters puts you kind of in the middle of the two, so I don't really have a good answer for that. You could keep doing it until you end up with an off batch of beer, but who wants to screw up a batch, right? I guess just experiment and stay within your comfort zone. If you really want to stretch it out and not have to worry about any of this, you could grow up a bunch of yeast from the original vial and freeze it. That's what I do. Check out the link in my signature. Hope this helps.
