As I understand it, in the RFC a 10 day leave usually came up after about three months of combat flying, although it might be deferred if there was a push on or if the unit had spent some time on home defence duties (e.g 46 and 56 Sqn). Short leaves might be granted as the OC Squadron saw fit, particularly if weather was poor or operations light. Then, of course, there could be convalescent leave after an injury. Most British officers headed for England. Colonials were slightly more inclined to see Paris on one of their leaves. For most British, however, the place was too full of Frenchmen.
The RNAS scheduled pilots off one day a week as a routine matter, and allowed home leave after about three months. Its officers frequently got home by destroyer instead of a plodding leave boat.
The Luftstreitkraefte seems to have been less regulated. A home leave of a week or two was possible after a few months at the front, and successful pilots were often allowed time off to attend things like aircraft trials, investitures, aircraft manufacturer-sponsored junkets, and other PR activities.