Just waiting for the wing to fall of a Nigerian PA-31 and the worldwide fleet to be grounded until a SIDs program goes in. You'll hear some screaming then...
The PA31 already has a limited spar and airframe life dependant on model. Depending on various mods/inspection to the spar over the life of the aircraft the PA31 is basically retired from commercial service between 20-26 thousand hours. If you compare this to larger aircraft which usually have service lives of 50-100 thousand hours its only a very small number.
The CASA retirement schedule assumes all other non assessed/time limited airframe components will last at least 2 spar lives so the airframe is limited to this in general without further evidence.
The PA31-350 has a spar life rated at 13000hrs, this gives it a CASA retirement limit of 26000hrs.