I also have a AIS SIM in my inexpensive phone I use only in Thailand. I've had the same number for two years, doing what xyzzy does. I've discovered that when my 30 days are up, the phone number is still active and people can call me but I can't make calls. I get a message saying that I need to top up to make calls. I know that I have gone at least up to a month with the phone in this status before topping up to reactivate my ability to make calls. I don't know how long you can go like this before you finally lose that SIM's number altogether and need to get a new SIM.
Also, anyone in Thailand with your phone number can top that number up for you before you arrive. This can be handy as you have a fully working phone the minute you touch down in Thailand. Of course, if you email someone to top up your phone for you they will logically assume that you will see them on your upcoming trip. There is "explaining" to do if this turns out not to be the case. It's not important for me to go into further details about this...