Hi 023 ,
Normally refineries and chemical plants ,operatiing life is taken as 20 years,when making feasibility studies,that is about 5% depreciation on equipment cost .
These values can be found in Process Plant Economics,by Peters& Timmerhaus- McGraw-Hill.
Real times,will depend on maintenance procedures,and some refineries have been operating for over 50 years! ,with high maintenance cost,and is more feasible to the owner than buying a new plant.
Regards,
Whylie