If I arrived in Thailand with a multi entry non O visa when would I need to apply for a retirement extension? Could I do it at any time or would I have to wait the full year until the O visa was nearing its end?

If you have a multi entry O visa that allows 90 day stays you can do it during the last 30 days of any stay.

If you have a multi entry O/A visa (qualified for retirement in home country) you should receive a one year stay and should extend a few days before your permitted stay ends.

Thanks, lop. It would be a multi entry O. Is the last 30 day thing a 'rule' or just advisable.

It is highly advisable. Whenever immigration is asked it is what they say. Some have done earlier if all paperwork was ready but there is really no reason to buck the system.

Ok, thanks very much.