How often are drop tanks actually dropped?
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.