It's nice to always have a job and not be floating out in the ether waiting for whatever the next big thing is. So, in that way I hope there's no a shelf-life for great shows. On the other hand, you don't want to be working on something that's reached its peak and become irrelevant.
I think for me, when I'm looking at a script I really try to consider what experience am I embarking upon, because for me it's really about the experience.