What are the validity and duration rules for a 90-day Non-O visa in Thailand?

Your first 90 day report is from when you arrive in the country. If retirement visa is technically 15 months. It's the 90 days plus your 12 month visa.

O-visa, give you 90days (3months) to enter Thailand, when you land in Thailand you get 90days (3months )when you show your e-visa to immigration control, then on the last month of the visa you can get an extension (one year if you qualify for it),so if you don't show your (o-visa which is not stamped in your passport now ) e,g, e-visa, you'll get a 60days visa exemption stamp, so what did you do at immigration control centre??,,,

Your visa has to be used within the 3 month period. Your 90 days will start at the point you enter thailand in April

Visa is issued and you must use it to enter before June 7th. You then get the 90 days duration of stay.

I thought Tom said his visa is valid until June 7th (expires on that day).. but you say Brandon that he must "enter" by June 7th (valid from that day) ?

The 90 days begins from the day you arrive. The expiration date (7 June) is the day you must enter Thailand by, in order to receive your 90 days.