Chiangmai immigration told me I could apply for a visa extension up to two months in advance. This in very useful in two ways. it gives plenty of time to get futher documents if required and if you are going to be abroad at the expiry date you can renew it before you go.

If you renew it 2 months ahead they still give you a one year extension from the expiration date. This effectivly give you 14 months before you have to apply again.

I have done this in Chiangmai with no problem but I no longer live there and will have to apply in Hua Hin. It seems that different immigration offices apply different rules and they change with the staff changes. Any one have experience in this matter of application date?

The general rule has been 30 days early at most places unless there are reasons for doing earlier (such as travel). That said under the new diversified system some offices have been suggesting earlier applications so believe it might be worthwhile to ask Hua Hin (if nobody has current information).