During the peak summer I would say a maximum of three round trips. As an imaginative example, the likes of Monarch or Britannia might have flown MAN-PMI-MAN-ALC-LBA-ALC-MAN with a single aircraft between breakfast time and the small hours. Doesn't give much down time for maintenance, mind you. I remember hearing on a BBC Look North documentary circa 1984 that a 737 holiday jet needed a minimum of fourteen hours flying time every day just to cover its costs.
WHBM may be able to provide a more precise answer, if he's reading this thread.