my non O marriage visa will expire on the 16th of july, When should I apply for the first extention ?

I though it was something like 3 weeks before the visa expires but someone told me to go 1 week before the visa expires

By the way, this will be in Koh Samui.

Edited May 25, 201511 yr by pokerkid

You can go anytime up to 30 days prior. Go early rather than later in case there is a problem.

As said------------ 30 days prior to the expiry of the visa .

You don't lose anything by getting the extension early. They will add the extension on to you current permission to stay. It's better to get it done early, as suggested 30 days early is OK, just in case you encounter a hitch in your paperwork that needs correcting or the car has a flat tire or whatever. If the extension is based on marriage you'll be given an under consideration period so you'll need to go back again to get the extension stamp.

my non O marriage visa will expire on the 16th of july, When should I apply for the first extention ?

Visa expiration has nothing to do with extensions. Since this will be your first extension of stay, it will chain to your current (probably 90-day permsission of stay) stamp (unless you have gotten a 60-day family extension). The 30-day before information given pertains to "before" expiration of current permission of stay, not expiration of visa.

You probably knew this, but others have mistakenly believed that extensions somehow depend on visas still being valid. Many reading this, I'm sure, have years of chained extensions emanating from a visa that expired a decade or more ago.