My wife wants to spend Songran in Phetchabun (we live in Pattaya). How soon before departure can you buy a ticket?

As early as you like, as soon as you know the date you want to travel go get the ticket.

Buy early as there will be many many many people returning "home" for Songkran.

How soon to buy a bus ticket

- - - before you arrive in Thailand

Get it purchased as early as you can,this route to Phetchabun is served by two operators,of the so called VIP variety and is notoriously busy at Songkran as are most routes in the kingdom.

I always use Phetprasert.

http://www.phetprasert.com

If you are thinking about it your are already a day late and a few baht short!?

Probably sold out already,as are the bus tickets from Bkk to Isaan.

I managed to get a ticket out of Thailand to bali for 350 dollar return

As soon as possible. All the trains will be booked up in a couple of days time and most of the buses as well. If you already know dates get the job done real soon