Hmmmm, good question. I believe I started looking about 2-3 months before I was actually ready to buy. I wanted to buy the best my money could buy, and what was best for me.....soon after I bought my guy, actually that night, I got an email back from an Andi/Morgan cross filly that I was in LOVE with.....but whats meant to be is meant to be, and I wouldnt trade my little guy for the world!
If you do start looking now, just let the owners know that you can't buy right away, and just to let you know, be aware that looking this early usually means that the horses your interested in, will probably sell (or might not) before your ready to buy, this happened to me several times.
And like other people have said, do as much research as possible, even if you have been around horses for quite some time. I spent 15 years or so with horses, buying and owning, to me anyways is a completely different experience!! Good Luck