[Boatanchors] long-term preservation

[HL]

Quick question: We all have vintage boatanchor items that sit on the  
shelf month after month.
I assume they should be powered up periodically--perhaps for about an  
hour or so--to preserve capacitors and other components.
But, how often? Once a week? Once a month? Once every six months?
Does it make a difference if they are low-voltage solid-state systems,  
such as 1970s HP and Tektronix testers,
or high-voltage vacuum-tube radios?
73 Hal KK6HY