To upgrade a separate power supply?

Depends on the voltage rating of the caps, what voltage is applied to the caps, and the ambient air temperature surrounding the caps.

My 40 year old Sumo Andromeda amp has been in daily use for six years with no issues! Designed by the great James Bongiorno of GAS Ampzilla/Dynaco 400/SST fame.

On the other hand my Brown Electronics Lab BEL 1001 died after a few weeks use! Needs new power supply caps. It too was built in the mid-80's.

Or, how many years since it was last turned on? If it's in frequent use and working correctly, it may not need replacement electrolytic capacitors. 

After how many years should the caps be replaced.?

Look here

and here.