They need a proper revision timetable, and to work out how much time each subject needs, to work out how much to do in a day. I would recommend a full school day, most days, so say 5-6 hours. Start early, lots of breaks, maybe 7-7.45, breakfast, 8-8.45, break, etc, but make sure breaks are stuck to rigidly, if you break when expected you learn a lot just before the break, but if you give yourself an Off-the-cuff break, you miss out on that particularly receptive time just before the break.
Make sure each chunk of time has a specific aim though, and lots of encouragement, any time is better than none.
Every day, review what was done the day before, briefly, and every week, review what was done the week before too, never only look once.
It helps to revise in silence, as it will be recalled in silence, ( do not listen to anyone who says they work better with music on! They are deluding themselves!)and it can help to revise during the times of day you will actually be sitting exams.
The only exception to that is quick reviews through what was studied earlier, in slots of a few minutes, in ad breaks, gaming breaks, waiting at the bus stop, etc