The Commission on School Reform think tank had previously claimed six hours would be needed to repair the educational damage.
But a new report from the group, which is made up of education academics and is part of Reform Scotland, says further school closures mean extra catch-up work is needed.
Lindsay Paterson, professor of education policy at the University of Edinburgh and a member of the commission, updated the group’s research from July last year.
Prof Paterson suggested eight extra hours of tuition per week for two years, or five hours per week over three years.
This would be completed by extra hours lessons in the later afternoons, he said, while also calling for additional funding.
Prof Paterson said: “Children in Scotland will have lost around four months of schooling since March – that’s around 40 per cent of a normal school year.
“Although thousands of teachers have been providing good quality online learning and parents have done the best they can, the closures have led to a massive loss of learning.
“Many children will have actually gone backwards because such interruptions can lead to them forgetting what they have learnt before.
“With an end to closures now in sight, it is time to turn our attention to how to recover the learning that has been lost.”
He continued: “We estimate it at 16 weeks and have suggested an eight-hour extension to the school week for two years, or a five-hour extension over three years.
“The Scottish and UK Governments are to be commended on the significant sums of money they are spending on the Covid recovery.
“There will be no money better spent than on ensuring our children’s education does not suffer.”
