When does the validity of a 90-day non-O visa start?

Just unnecessary additional info as many people are mixing up visa validity and entry stamp.

As Lloyd said, the visa itself's validity of 90 days (<- the period you can use that visa to enter Thailand to get an entry stamp for the status on the visa) starts on the issued date but the Thai embassy/consulate.

Your 90-day stay on Non-O starts on the day you enter Thailand within the visa validity as day 1, even if you enter Thailand on the last day of visa validity.

It will be valid for 3 months from the date of issue