He will continue till 5 AGMs
Even if it takes 8 years for 5 AGM to complete 
Day 5 AGms happen he will retire 
Rotation is given in years but it should be applied as 5 AGMs
Above logic is for date till which he will stay as auditor 
Work wise he will do audit of 5 financial years and next auditor will come and do remaning work of 3 years, which has already happened 
 
 
There is no clarity what will prevail whether rotation under 139(2) or automatic reappointment under 139(10)
My opinion is strict provision should apply , he should retire as per 139(2), next auditor appointment will be as per instructions of authorities