I had a player retire on a 30% chance, ironically just after I renewed his contract, which you still take the cap hit on.
It doesn't really answer your question, but if you do have a player that does have his retirement chance creeping up, LESS playing time can prolong his career, eg: take him off being a starter and maybe STs as well.
I think the retirement risk has an element of chance as well, in other words there is no formula to a player playing on more than another.