I usually switch up the brand I use per bottle. As for the shampoo itself, if it starts separating or smelling weird, I'd say it's time to throw it out, but I'd imagine that it would take years for that.
I've heard that using the same one for too long makes your hair get used to the particular stuff in the shampoo, so switching it up can refreshen it again. But I've also heard that this is just a myth too. So I don't know for sure on that one.
But getting a different brand each bottle has me switching every couple of months, so I suppose that's good enough just in case, and it doesn't really bother me to pick up a different kind each time. Sometimes I'll try out a new one to see if I like it. Otherwise I'll just rotate amongst my tried and trues.