I've tutored 11+. Check with the schools if they publish past/sample papers. If they do, get hold of as many as possible. Any kid who is familiar with the format of the exam will be at a huge advantage. Get her to do one timed now and see where she is (I would tot up the mark so you know where she is, but don't tell her - if it's lowish, she'll be anxious, if it's high, she'll be complacent!)
Then work out from her paper what you need to work on. This isn't necessarily the content of the subject. Is she racing through maths, not showing working and making silly mistakes? Or is she taking ages over her creative writing? you get the picture. Exam technique accounts for so much of a mark - be up front with her about that, so that she understands the preparation. It's up to your judgement of what would suit her how much you do, but get her to do chunks of past papers regularly, maybe 2 or 3 a week. If you can't get hold of any, or you want to keep them for a dry run at the end of the summer, do the Bond 11+ book. If it's not a battle (and you say she wants to do the exam, so hopefully not!) then it will build her confidence hugely. Summer is the best time to do this because they're not tired from school.
Then spend the summer building on things in small ways. Numeracy is fun and easy to work on while you're out and about, practising times tables, both multiplication and factoring, so that she has lightning recall, would be my priority. Creative writing and comprehension also benefit from little and often and can be fun - get her to read a newspaper article here and there and chat about it, or get her to keep a summer diary, or write her own little novel, whatever she fancies. Essential is that she reads a lot, so stock her up at the library on her favourite fiction (it really doesn't matter what it is, within reason).
Verbal and non verbal reasoning are the ones that benefit from a certain number of hours just sitting down and practising. so I would do a bit of that, maybe once or twice a week. The Bond books are very good for those.
I hope that doesn't sound overwhelming - to recap, I think she can be sitting at the table maybe 2/3 times a week to work on practice papers (use this time to cover the subject content of maths, the basic geometry and algebra), and the other days you can be sneaky and work maths and english into what you're doing. Mental arithmetic and creative writing benefit from little and often over time, whereas verbal and non verbal reasoning skills can be built in a couple of weeks with solid practice if needed, it really is a skill that is taught. And every day - before bed? - tucked up with a favourite book for half an hour. No need to discuss or anything, just let her enjoy - it will build her vocabulary. The days you are at the table, I would do 45 minutes - an hour (as opposed to 20 minutes a day) - she has to get used to working for that long to sit the exam.