I hope 0% because that would be such a shame, even more so then Ware. I can't imagine Witten leaving before Romo (if he's still being productive), I think there's a great chance they'll retire the same year. If I had to guess how many years Witten has left, I'd say 2-4 years.
Jason is so good that even as he loses speed and is slower, he still gets open. He's a TE, he can probably play for the next 15 years if he so chooses.