How long is the NON O e-visa valid for after approval?

all information about your visa is written in your visa ! Including how many days you can stay

And there’s the answer.

A single entry Non-Immigrant type visa is valid for 3 months from the date it's issued.

Print out the PDF they emailed you <- that IS your visa and you need to give it and your passport to the officer when you stamp in.

LOOK on that PDF and it should have a date of issue and a must be used by or valid until date on it. <- that valid until is the last day you can enter thailand on that visa and get stamped in for 90 days