We bought ours last year 6-8 weeks before thru Ares travel.
The risk of waiting too long is that there is a price increase. You also risk not getting them in time if you wait way too long.
The risk of getting them early is if you have something happen so that you have to cancel the trip - son breaks leg or something. The tickets are still good for a later trip, but you will have to take that trip. You also risk missing out on any special deals that might come up.
I thought 6-8 weeks was a good tradeoff for us.