I have a Cornell cap, with a value of 490,000uF that I just now grabbed off my shelf and discovered it had
20v of charge still on it. I seem to recall reforming it about 18months ago, in anticipation of using it
for some forgotten project. It is rated for 40v. I got it surplus NOS from a weapons lab, an I have no idea of
age or how long it was on the shelf before I got it. Do electrolytics normally hold a charge for that long?

 

They can. With high voltage caps like rated at 10 kV, it can be quite a surprise. Ehey should be stored with keepers on them.
Rough numbers 0.5 F, 10 M, 5RC is about 289 days to 98% or so.

 

That might not be normal but not unheard of, I just now checked a 10,000 uF @ 10v & found 6.9V
remaining. Not sure of when last used but many months.