16-year old Trane central AC one package unit is not cooling well?
16 years is good , it would be hard to give an exact # but based on the size and the age of the system even I could give an idea based on the size and it is still functional, 5 ton 10 lbs R22 simple top off at this age if a couple measurements prove the system is healthy and only needs a charge that may last 7 years , I base future at 1/2 if only low on charge bigger system more refrigerant smaller system less.