Won't give you numbers, as my numbers are irrelevant, based as they are on different instruments and subject.
BUT, I did have an experience that taught me a lot:
My students were almost exclusively of the type who faithfully came to lessons each week, and did so for years and years, with good results.
Then, I "bought" another music school, inheriting its pupils, and suddenly noticed in them a lack of scope, lack of commitment, bad practising and bad results.
It took me some time to realise what the difference was, and it was that that other school had taken in each student it could get, and had not made good, firm deals ("contracts" as it were) with them - - thus allowing for all kinds of slippery slopes, including the lack of building up good practising routines. This of course had had a negative effect on the effectiveness and, more importantly, on the fun.
This experience affirmed my belief on how important the time spent before the first lesson is, and how important the first lessons are. If one wants students who stay, and with good results, this IMHO is at the core of the center if the middle of what's important.
Be very friendly and positively supportive, and paint a rosey perspective for them, but also be very strict with what you demand from them, and do not just take any student you can get.