Don't start too early otherwise you will drive yourself mad with considering options for more than one year (or two at the most)
It is a very, very stressful (and it can seem hideously competitive) time and if you start doing it too early in your child's primary school education you are just going to be totally burned out by the time they get to Year 6 (although I stand to be shouted down by people who might consider themselves not to get ruffled!).
As Blu said, Yr 5 is really early enough to start going to open days but it does no harm to possibly read a bit about potential school options
before they get into Year 5. Certainly for the grammars, the open evenings tend to be in the summer term so if you don't start thinking about options before Year 6 starts you are slightly snookered as you won't have visited the school and might even have missed the 11+ application form submission dates....
The other issue is that if you have a lot of schools on your longlist, it can be extraordinarily tiring fitting in all the visits into a few weeks in September/October. Some of the open evenings will clash and then you have to try to juggle daytime visits around other (most likely work) commitments; some schools offer loads of potential visiting sessions but others don't. The children are allowed to miss school to attend daytime visits but they get totally tired out, being bombarded with so much information etc..... By the end of October everyone is feeling very frazzled - and it's worse for the ones who are simultaneously sitting 11+/selective exams (and doing extra work for those exams) and doing school work too!
I think that if you have so many different options, you could very usefully think about those and which ones would best suit - otherwise you really will have a lot to contend with....
We only considered state schools although DS was pretty sure he really wanted to go down the grammar route. But we probably did visit about 10 schools in total and he ended up sitting five lots of exams - we were just thankful, come the end of November, that we could relax (until Offers Day on 1/3/2012). Some of DSs classmates are sitting a private school test too in January which means five months of exam stress.
Sorry, don't want to put you off but do not under-estimate how tiring it is for everyone in the family for the time the whole choosing process is going on - even our DD (who is nowhere near that stage yet) breathed a huge sigh of relief when the visits/exams finished and the CAF form was submitted.
Good luck!