I'm about to book an Air Asia flight BKK to Yangon but I don't know how long I want to spend in Burma. Will they let me in on a one way ticket or do I need a return ticket?

yes-no
BUT its FAR better to book return at a likely date and use the changedate option if you change plans. buying tix in Myanm will cost $$$$more, and dont expect to be able to do that yourself via its instable web/net

Yes they will let you in on a one way ticket, but as bmta said it will cost you more dollars to buy one there to return. But what the heck, if you are unsure how long you want to be there, then this is the best way to go. Plenty of places in Yangon to buy your return, and they will insist you have US$'s so don't overlook that. Make sure you have plenty of good, clean US$ and you will be fine.

bmta, how easy is it to change flight dates from within Burma ,given I plan to arrive back in Rangoon the day before our flight, which is a Sunday? (6th Jan) .For that matter ,how will I be able to find if our flight has been altered , as supposedly internet is not so reliable in Burma.
With thanks Norm.

Flights with airasia can be changed up to 48 hours before the flight, at a fee. Trouble is that you only get credited what you paid for your flight initially but have to pay the, now much higher fare for the flight a few days away.. It's not really a good way to go.. better to not book in the first place.... or stick to your original booking....
Internet usually works well enough to check bookings.. otherwise use the telephone.

Oldbaldy.. I recall that you were asking about the AirAsia flights a while ago.... It looks to me as if AirAsia are planning to go to daily flight from December 1... I just checked their Sunday flights through all of December and they are shown as being available.... Time will tell, but I think you'll be fine.