If you believe Alec Baldwin, and there is no reason not to, next season will be the end for 30 Rock. Alec had already said he was not going to sign again when his contract runs out next year and now he told New York Magazine that Tina Fey will also not be returning. So, that would probably spell the end of the show. I guess the rest of the cast could stay and I think it would still be pretty funny. With the exception of Tracy Morgan it is not like any of the other actors are being hounded to come work on different things.
I have thought before that Alec Baldwin has always kind of kept the show together, but in the past season or two it is like they have taken his character and made it more and more cartoon like and unbelievable. It has been a good run and a great show.