We have found frequency required tends to correspond to where learners fit in terms of the attainment spectrum of their peers.
e.g. learners below the lower quartile, (below the 25th percentile) need to do timely practice at least 3 times per week, with no more than 3 days gap between sessions e.g. Monday, Wednesday, Friday is ideal (where the gap Friday to Monday is 3 days), but e.g. Monday, Tuesday, Thursday will work for most learners. For those learners, where you find such a large gap won't work, can have an extra 5 minute practice scheduled, perhaps in a registration or with a parent or carer at the weekend.
e.g. learners between the lower quartile and the median (between the 25th and 50th percentile) can do timely practice twice a week, but three times is better. Again it's the gap between sessions which is important. e.g. Monday and Thursday has gaps of 3 and 4 days, this will usually work, but e.g. Monday, Tuesday has a gap of 1 and 6 days so is unlikely to work for some learners. For those learners the too large a gap doesn't work for, scheduling in an extra short session might work. For learners in this ability range setting homework can be used to give the 3rd point of practice - usually just 3 or 4 questions will bridge the forgetting gap.
We have understand that about 70% of learners at or above the median will remember new learning for at least a week (see RohrerTaylorANDtp.mp4), so once a week may be sufficient, but again a class session and a homework session timed to bridge the gap, should make sure no learners are left behind.