We didn't start work for the 11+ until year 5 (January) so essentially had just over 10 months' worth of preparation.
My girl went to a (recommended) tutor for 1 hour per week. She attended an 11+ club at school on a Monday for 1 hour. We did the homework given by the tutor (although sometimes she just had had enough of it all) and, from the start of year 6, her teacher gave the 11+ children specific homework every night pretty much.
I was quite worried that (prior to the exam), she just hadn't read enough books (enjoys reading, but not an avid bookworm) and was actually quite worried how she might fare in the English. I did also get her to learn some vocab a few months before (not a single word came up apparently in the exam lol!).
She was given the Bond Books to do in the summer holidays (2011) by her teacher at the end of year 5. She wasn't terribly impressed about being given this and completed half of the English, about the same for the maths and all of the VR.
That's what we did and to be honest, that little lot was enough for her. If I'd given my girl much more than that I think I would have put her off completely as I know her limits. I think though that it's fine to start tutoring earlier than I started, as long as you take it very gently to begin with.
As parents we know our DC's limits and to go beyond that is just not acceptable - so, each to their own basically. One horror story that has recently come to light is regarding a child my daughter knows - his mother made him do 4 HOURS worth of 11+ work EVERY NIGHT after school for months. Outrageous and verging on child abuse as far as I'm concerned...
Again, we can only prepare our children so much as we all know that the 11+ is a tricky exam - even for the very bright kids. Other factors such as the limited time they are given to complete each paper, stress, nerves etc etc all come into play on the day.
Very glad I only needed to go through it once...