You can be lucky and they keep working, or like in my case the Weston Master V cell gave up. Many Weston's sold second hand have cells that are on there way out.

I use a Leica meter, which I want to replace, the Leica dealer I use won't sell the ones with selenium cells at all because hesays they are nowhere near accurate. Mine is but it's damaged and temperamental.

You run the risk of not noticing your meters failing so it's important to check it every few months, (about twice a year).

Ian