That seems like a lot of revision to ask for.
On the other hand if she is overwhelmed and doesn't know where to start then she definitely needs to do some just to learn 'how' to revise. My DD1 needed handholding through revision as her dyspraxia meant she couldn't organise it herself.
List the Subjects
List the topics under the subjects
Estimate how long she needs to revise each topic
Add it up
Work out what you both think is reasonable revision time over the holidays. e.g. average 2hrs per day for 15 days = 30hrs total. Maybe add in any other time you think will be available before the exams start.
Now compare with estimate of how much revision is needed. Alter allocations or revision time until they match.
Look at plans for the holidays. When is revision going to be done? 9-11, or 4-6, or when? If some days are booked or you want a solid break, then need more hours on the other days.
Allocate subjects per revision session, probably no more than 1hr blocks, maybe 30-40mins.
How do subjects need to be revised? Making revision cards or mind maps? Using online resources? Doing practice questions? Try to make learning active not just reading a text book / revision guide.
Have an 'aim' for each session. Not just 'revise chemistry' but 'learn properties of metals and non metals'. Then she will know if she has succeeded. For English not just 'revise Macbeth' but 'identify quotes to support views on 4 main characters' or 'key points regarding use of light and dark symbolism'.
Maybe some sessions can be 'discuss with mum' eg you have her revision cards for Macbeth, and she talks about themes, characters etc.