These topics is interesting because you really need to know what makes Kruger tick to answer.  
I suspect he has lots of money and that isn't much of an issue for him.  He has been a head coach of a major program since 1986.  
I would think the location of his children and grandchildren is a factor.
What he wants to do with his time is also a huge factor.   Does he want to travel the world with his wife?  If so, that is a lot easier to do at 65 than 75. 
Is his greatest joy in life teaching young people?  If so, he might want to coach longer.  
If I had to guess, I would say five years with no real basis for that guess.