How to Calculate the Depriciation of Steel Belt

On the face of it the belt is about a third of the way through its rated life and two-thirds the way through its nominal life. In what sort of condition is the belt (such things cannot be seen from here)?

IMO rated life of components is generally meaningless.
E.G. Brush life on Dc motors is rated as some ludicrously short figure because the manufacturer is scared of units being returned as faulty.

(I can't remember off hand , but I think it was something silly like 5000 hours)
About 15 years ago we went from AC to DC motors (small 12v 1A) I have never yet seen a motor with worn out brushes.

What I'm saying is inspect it once a year (or more often) and shedule in a replacement when you think it's nearing the end of life.

The numbers are there to try and quantify the reality... not the other way round.
Del

Bear in mind that depreciation is a way for an Accountant to set aside monies free of local taxation to provide for the replacement of its assets at the end of some time period. The job of an Engineer is to consider carefully the balance between repairs and renewals with the aim of maximsing the use of that asset during its life. Those two approaches are not incompatible.