I did my PPL on an Air Cadet Flying Scholarship so young and fit in those days, regular days were three trips, I did the odd day with four and that was definitely over the top, so maybe TRY a three trip, but be ready to cancel the third trip the next day if it is too much! As for back to back trips, do not even go there, if I fly a trip with a student over an hour airborne (not including long Nav trips where nothing new is being taught, just practising what is already known) I can see a rapid deterioration in performance.