About a year before they sit the 11+ (some people say longer). I've seen them charging up to £35/£40 per hour, but there is quite a lot of variation depending on the tutor's experience and track record.
I think that if parents decide their child needs 4 years of tuition, then they need to ask themselves if that child is really suited to a grammar school. But if you have the money, time and energy then, yes some people start years and years in advance.
I would say for 11+ start yourself when your child is in yr4 with adding a little bit of extra work on top of homework. Not masses just a bond paper here and there, so when the intensity kicks in in yr5 it's not so bad.