Personally, I don't think any physical limitations will curtail Stafford from playing until he is 40 or 41 years old. He has shown NO diminishing of arm talent, or of his mobility. And his mind has done nothing but grow as he dissects the D.
The issue will be if he continues to have the fire in his belly, and if his family is at peace and encouraging with his continuing to play. I think at this point, even though he will continue to demand the highest salary on the team, he will not demand to be the highest paid QB in the league and as long as the number is respectful and in line with other NFL starting QBs of his level, salary will not be an issue. He is too rich and too smart not to know that if he takes up too much salary it will be near to impossible to put a good team around him.
