It's never too early. But is your purpose to save money or some other worry?
I've purchased tickets in-person at the counter a few hours before the plane departed. Also no problems.

The only problem with buying them so early for your (I'm assuming) vacation, is that you are then pretty much locked into your plans. I usually buy my tickets when I'm over there...either online or with a travel agent. I might not get the best deal for booking a few days in advance, but the convenience factor makes up for it.

If you save 500 baht per flight by booking online, and you have 4 internal flights, that's saving/wasting 2000 baht total. 2000B / 30USD = $66 USD. In the grand scheme of the whole vacation, that's not much money.

Are you worried about the flights selling out or being cancelled? I've only ever experienced that once, and that was because the airline cancelled service to the city alltogether. They rebooked me on the other airline and my departure was moved back a few hours.