How long do re-entry stamps last for a marriage visa in Thailand?

A re-entry permit lasts as long as the stamp you have when you get it.

So if you're on a yearly extension from the immigration office based on marriage to a thai and you buy a re-entry permit it's good for that whole year.

For example; a multiple re-entry will let you exit/re-enter thailand for the whole year the extension is valid for and you will get stamped in until the end of the extension each time you re-enter thailand