What medirina* says.
With disorganised dyspraxic DD1, now y12, I did as follows for y10 & y11.
Went through with her each subject to list main topics. Got her to estimate how much time needed to revise each topic. Added up total time.
Discussed how much revision she thought she should do each evening and at weekends. Added up total time.
Compared the totals and adjusted/compromised.
Agreed when revision would be done (e.g. straight after school v after tea).
For a NT child that should be enough really. But for DD1 we also:
Scheduled in subject by subject into each revision slot, then before each slot agreed which topic would be addressed. We were flexible with swapping slots and topics, but made sure that no subject was being unduly neglected.
We also discussed how to do revision. Active revision - making notes or diagrams, doing practice questions, questions on BBC bitesize, is more useful than passive reading the book. Mix and match different techniques to find what works well and to alleviate boredom.
(Plus active techniques makes it easier for you to check that work has been done as there is something to show for it).