Question: on a multiple-entry, is it possible to do a border run several weeks before your 90 days expire? Or does it have to be within a certain timespan of the end of the 90-day period?
TLDR : Answer Summary
It is possible to do a border run on a multiple-entry visa whenever you like, even several weeks before your 90-day period expires. Upon re-entering Thailand, you will receive either 60 or 90 days, depending on your visa type.