It's only a year really - the entrance exam here is taken in December of Y6 so at most it's 4 terms from the start of Y5. FWIW, it's my understanding that their performance on the English paper is the most important, so if your daughter is strong in English she has a better chance than average.
Have never had mine tutored, partly from principle, partly from finances! This is a small country town with a grammar school, a very good secondary modern in town and another in one of the villages. They are all good schools but there tends to be a significant anti-swot element among the children at the town sec. mod., and an academic child has to have a strong character (or be in a group of like-minded friends) to overcome that.
Some of them get excellent GCSE results. However, when they move into the grammar school's 6th form, the ones that don't come from its lower school are slower to get going with the more independent form of study required at A level, even if they got sparkling GCSEs...the grammar school ones seem to be taught more than just how to pass exams.
On the other hand, the non-grammar schools here provide far better non-academic facilities - art, drama, mport, music, IT, tech, etc. (And good independent schools tend to have the best facilities of all!)
Good luck, anyway, whichever way you go.