On the civil side the legal definition is that you have to have exercised the privileges of a licence at least once in ninety days otherwise it is deemed to be suspended.
There is no other 'currency' apart from that laid down. It is becoming more common now for units to stipulate a period away after which you have to be 'screened' before being let lose on your own again. In my units case it's any period over 2 weeks (so normal leave periods tend not to fall into the category).
Bear in mind of course that most civil units may only be staffed for 2 or 3 positions at a time, not the multitude that mil units are established for (ground, tower, approach, director, zone, talkdown etc) so getting seat time isn't as big a problem due to the lower numbers of staff generally involved.