How long is the perfect lesson? A generation ago, secondary teachers expected periods to be timetabled in handy 35 or 40-minute chunks.
Independent school teachers are still likely to be planning lessons at those lengths - although things are changing - but in the state sector, timetabling varies from school to school. Some swear by 50 minutes, others prefer an hour, or even two.
And one school of thought holds that the 100-minute lesson is the way to go.
Unusually for anything to do with education, not only do schools get an entirely free choice on how long their lessons should be - as long as curriculum requirements are met - but no official research has been carried out into it. The changes are being driven by schools themselves, partly as teaching styles move away from chalk and talk.
Andy Schofield is head of Varndean School in Brighton, which experimented briefly with the two-hour lesson before settling, four years ago, on 100 minutes. The school, a specialist technology college, runs a fortnightly timetable with three lessons a day.
Varndean's teachers like the system, he says, but he gets complaints from parents who are teachers elsewhere.
"What we've tried to do for the past three years is to configure the school around what's best for learning rather that what's most organisationally convenient," he says.
"I'm not saying it hasn't had its critics. Those include some of our parents, particularly those who are teachers. I had one who said, 'I am a teacher and I couldn't do 100-minute lessons and I don't see how your teachers can'. But we put a lot of effort into professional development.
And think about Year 6. How long are lessons in Year 6? They spend all day in the same classroom."
He adds: "We do an annual survey of our staff, anonymously asking how they rate aspects of their lessons. By and large, teachers think their teaching is more effective and their planning has improved tremendously. They've only got three lessons to plan for rather than five and the kids have only got three lots of things to bring.
"Pupils come in, have tutor time, have their first lesson then break, second lesson then lunch, third lesson then go home. There's less movement between classrooms - moving around from one place to another which can take up to 15 minutes from a lesson. There's more time, in theory, to be spent on learning."
The Varndean School experiment came from a cross-section of its teachers working on an innovation strategy. "We used to have five one-hour lessons and some subjects always wanted doubles. We tried 120 minutes and some people found it was a bit long. Staff homed in on 100 minutes from a practical point of view," Andy says.
"Also, the research shows 20 minutes is the optimum time for listening to people talk, but there is nothing in the research to say people automatically learn in 35, 45 or 60-minute chunks. We felt that teachers - and it brings a wry smile when I say this to them - can wing it in an hour-long lesson, can be under-planned and under-prepared. You spend 10 minutes calming the class, talk for 25, set them an exercise and finish.
You can't do that with 100 minutes."
What is ideal for one school, though, is not for another. Varndean's 100-minute model is a bit of a rarity, with some high-profile schools, such as Thomas Telford in Shropshire, opting for two-hour lessons and a greater numbers of secondaries settling on 50 or 60 minutes. Though many independent schools, including Eton, are likely to have kept the "traditional" lesson of about 40 minutes, with double periods for some subjects, change is in the air there as well.
A quick and unscientific survey of 16 private schools in the North-west of England found a quarter had recently overhauled their arrangements.
Professor Dylan Wiliam, deputy director of the University of London's Institute of Education, thinks state schools experimented with timetabling in the 1970s and 1980s by creating double lessons of 70 to 80 minutes, which means less student movement.
"Teachers of practical subjects (including science) liked these longer lessons. Teachers more used to chalk and talk (MFL and maths) didn't. "I think schools operate a range of models, including four lessons per day (75-90 minutes each), five lessons per day (55-65 minutes each) and six lessons per day (45-55 minutes each). The optimum length of lesson depends on the subject. For some subjects, a short 'input' type lesson of 35 minutes might be fine, but it would clearly be inappropriate for art, PE, science practicals and so on."
Professor Wiliam adds: "The most important point is not the length of the lesson, but the appropriateness of the activities and the recognition of the need to change focus every so often."
Sue Kirkham, a head in Staffordshire, says there is no national consensus on the optimum length a lesson should be, but the literacy and numeracy hours have had some influence in promoting 60-minute slots. "Different subjects would prefer different lengths. With something like PE, it would hardly be worth bothering with 35 minutes by the time you'd got changed for the lesson. You do get divergent views and I think that's why quite a lot of schools have gone for one-hour lessons."
Sue, past president of the Association for School and College Leaders and a Qualifications and Curriculum Authority board member, adds: "The reason for changing has to be better outcomes for the children."
And schools have to be committed to the change, which affects timetabling, staffing and teachers. "Some staff are just wedded to what they are used to," she says.
"Planning a lesson is quite a delicate operation, especially in practical subjects where you have got to finish bang on time. You do get used to knowing exactly what will fit into a length of time. But it makes me smile when you get a member of staff who tells you they have always taught 50-minute lessons and couldn't do it any other way. They go for a promotion, move to another school where they're going to have to teach for an hour - and that's okay."
Andy Schofield, meanwhile, who is leading the Specialist Schools and Academies visions group, thinks his school has not yet been radical enough.
He wonders if the 100-minute lesson works so well in the afternoon and cites an American school which starts and ends the day with long arts lessons, with shorter literacy and numeracy lessons sandwiched in between.
"Maybe next September we'll be a bit bolder," he says