I agree with Rich’s statement that the time to retire differs from athlete to athlete based on a number of factors.  Although, I would add that the fighter’s style may even be the most important factor of all.  
As we age we see that certain things are “the first to go.”  For example, quickness goes before strength.  Therefore, a fighter that relies on quickness may see more drastic difference in his performance as he ages than a fighter that uses more strength ie. a wrestler / ground and pounder.
Thankfully, I see Rich as an extremely well-rounded fighter that lives right, trains hard and has a skill-set that should allow him to fight at a high level for a long time.  His 3 recent losses are a result of stylist problems, not eroding talent.
Matt