Hi Irish Guy,
Welcome to aviation!
To answer your question above, somewhere in the region of an hour and a half is long enough to have a good, constructive lesson, without being so long it leaves you feeling tired, jaded and unable to remember half of what was going on. That's what the practice qualifying x-country's for!
The best person to consult on this is your instructor, as he/she will be the best judge of your flying ability and capacity to retain information!
All the best and good luck.
