How often are drop tanks actually dropped? I know drop tanks are used to extend the range on many fighter jets, but how often are they actually detached from the plane once empty? A lot of WWII movies and documentaries show tanks being dropped before entering combat, but I'm curious about more modern planes and non-combat conditions. Does the increased air resistance offset the cost of dropping the tank on say, a standard patrol, or are they landed and recovered with the plane and if so, are they re-used or simply scrapped?
In general, unless it's an emergency or needed for certain flight operations, the tanks are retained and flown back to base.

There are maneuvering limitations for carriage of certain stores on some aircraft.  The F-16C/D for example limited maneuvering to +6.5/-3 Gs with stores on the inboard two pylons and/or centerline drop tank installed.  If the aircraft was attacked by an offensive flight of enemy fighters, one of the first items prior to commencing ACM with the threat was to jettison stores on these stations.  This allowed the aircraft to maneuver up to a maximum of +9 Gs as the fight commences.