I usually start looking the second that I know that I want to go. I started looking for my upcoming trip (in August) around February. The cost of tickets were outrageous, so I was hoping that with time the cost would go down. then the cost went up, and I was upset at myself for not buying them, but 8 weeks before my departure, I found tickets for a better price than I was actually hoping for.
So, like was posted previously, I buy when I think the price is reasonable enough for me to buy.
Jo