Schools will be able to bid for funding for extra activities like sport and art, with at least 25% of secondary schools covered, he has said.
Mr Osborne's eighth Budget will include £4bn in extra spending cuts, and transport investment announcements.
He will also pledge that every school in England will be removed from local authority control.
Schools will either have to have converted by 2020 or have an academy order in place by then, so that they are committed to converting by 2022.
For any school that fails to have a plan in place, the government will take on radical new powers to intervene and ensure academy conversion takes place.
Academy status, introduced by a Labour government, was originally reserved for schools in urgent need of improvement, but since 2010 schools have been encouraged to convert and have been given extra funding for doing so.
Currently, 2,075 out of 3,381 secondary schools are academies, while 2,440 of 16,766 primary schools have academy status.
The plans will not apply to Scotland, Wales and Northern Ireland, where education policy is devolved.
After-hours activity
Malcolm Trobe, interim general secretary of the Association of School and College Leaders, said there were a number of issues around the idea of a longer school day that made it "complicated".
Classroom
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Heads say schools already offer more than standard academic lessons
"Lots of schools do a lot of high quality after-hours activity, including revision classes sometimes in the school holidays," he said.
"It's going to be quite complex to define the difference between those activities that schools are already laying on for pupils and any additional activities which come out of the Chancellor's Budget statement.
"We also believe it's highly divisive to have these funded activities available in 25% of schools - potentially youngsters in some schools would be in an advantageous position over others."
Head teachers said schools already offered a wide range of after-school, and holiday, provision.
Russell Hobby, general secretary of the National Association of Head Teachers, said any extra funding would be welcomed.
"The idea that most schools shut at 3.30 in the afternoon is itself pretty outdated, but we have no problem with extra money to help them in the activities they offer.
"Any expansion of hours must be properly funded, as school budgets are extremely tight. There's not a lot of evidence internationally linking a longer academic day to higher standards, but extracurricular activity is usually valuable.
"As long as this remains at the discretion of schools to meet the needs of their pupils, then it seems positive."
'Best start'
But Mr Osborne said Wednesday's Budget would "put the next generation first", with a "bold plan to make sure that every child gets the best start in life".
George Osborne
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Mr Osborne says he wants to "help the next generation"
"It is simply unacceptable that Britain continues to sit too low down the global league tables for education," he said.
"So I'm going to get on with finishing the job we started five years ago, to drive up standards and set schools free from the shackles of local bureaucracy.
"I also want to support secondary schools that want to offer their pupils longer school days with more extracurricular activities like sport and art. So we'll fund longer school days for at least 25% of all secondary schools.
"Now is the time us to make the bold decisions and the big investments that will help the next generation, and that is what my Budget will do."
Scotland, Wales and Northern Ireland - where education is a devolved matter - each have their own systems.
In Wales, school governing bodies set their own start and finish times - although local authorities can override them.
In Northern Ireland, the law states that the school day must be at least three hours for pupils under eight, and four and a half hours for pupils over eights.
In Scotland, education authorities set hours, but the school week is commonly 25 hours for primary schools and 27.5 hours for secondary schools. Scottish councils were banned from cutting the length of the school day to save money in December.