I noticed a debate on a forum about the best lesson duration. Someone has posted that pupils can’t concentrate for more than 40 minutes and suggested that that is the best 
lesson length. The same person has subsequently posted that two 1 hour lessons is better than one 2 hour lesson.
For a start off, every learner is different. Some have difficulty concentrating for more than five minutes, and you can’t help but wonder if these should be driving at all, because they’re going to have the same issues when they’ve got a full licence. However, the majority of pupils have no trouble doing a 2 hour lesson, and to suggest that they stop learning after 40 minutes is absolutely stupid.
Sure, if you spend an entire lesson repeating the same thing over and over again – and unfortunately, many instructors do structure their lessons like that – then yes, the pupil is having to concentrate hard and it’s highly likely that mental fatigue is going to creep in very quickly. It’s not the pupil’s fault – it’s the instructor’s, for not conducting their lessons properly.
Taking the manoeuvres as an example, I usually do not like to spend more than about 15 minutes on any one, because I’ve found that after two or three attempts other mistakes (like missed observations) start to creep in, and any improvements are overshadowed. Not with all pupils, but certainly with a lot of them. If we have one try, and it’s better than 
last time, I leave it and move on to something else.
When I pick a pupil up for a lesson, frequently they live somewhere where it is not possible to cover whatever it is we’re going to cover. So we have to travel – to find roundabouts, busy roads, dual-carriageways, junctions, an industrial estate, and so on. It can take 15-20 minutes driving on very familiar routes – boringly familiar, if you just stay there – to get to the trickier ones. Then, and only then, can the lesson begin properly. The leisurely drive to and from the location requires very little concentration compared with that required when dealing with something new or difficult, but it allows consolidation of previously learnt skills (especially when other road users provide new experiences) and is valuable in that respect.
Of course, once you get to your intended location, blasting them with roundabout after roundabout, or big junction after big junction, requires a heck of a lot of concentration, and doing that for more than 10-15 minutes without a pause is just asking for trouble. Again, it is down to a proper lesson structure to make it work.
If a pupil just books 1 hour lessons, for many that will only leave about 20 minutes to deal with the main lesson topic once travel to and from the relevant locations is taken into account. If something else happens along the way and you have to stop and discuss it, you’ll have even less time to cover the intended topic. But if they book 1½ or 2 hour lessons then there is plenty of time for a leisurely and constructive consolidation drive followed by a bit of hard work.
For most pupils, a single 2 hour lesson is much more constructive than two 1 hour lessons. In cases where it isn’t, then don’t do them!
And let’s not forget the fact that as ADIs we’re running a business and not a charity. Lesson duration has to be tailored to satisfy some of the instructor’s needs as well as the pupil’s. Longer lessons require less travelling distance between them, so if there are no other issues to consider (like those with poor concentration or limited finances) then longer lessons are better for all concerned.