Tam there are no hard and fast rules as to how long a stallion will remain active and get good progeny. It is often the case that the very good stallions tend to get smaller and smaller books as they age, mainly because their own sons are getting all the good mares.
For example, Sadler's Wells book started to fall after Galileo's success became apparant, falling from 124 in 2005 to just 64 by 2007, his final year (he stood in 2008 but failed to get anything in foal and was retired).
The same can be said for the wonderful Green Desert, his sons at stud include Invincible Spirit and Cape Cross so it is little wonder that the old man's book has now fallen to less than 20. At the age of 28 he is still active (and looking great by all accounts) but his fertility has been on the wane and, I believe, he failed to get any mares in foal this year.