I have just done an ARC renewal on a C177, as far as I could tell there was no life limit on the seat belts. There is on the C182, as we know it is 10 years, notified on a temporary revision. The C152 on the other hand, no lives on the seat belts I have checked the manual & the TR's.
On the 177 BTW the belts were in perfect condition & in a colour to match the interior, I bet you could not match those again.
The trouble with EASA & part M, If you ask me, they are trying to take away the engineers role of decision making. In the past, belts in good condition, leave them. Belts frayed change them. Simple.
Now they do not trust us to make our own decisions, if the book says change it, then change it. In the past the manufacturers lives were a recommendation not the law.