How soon can I apply for my Non-Immigrant O visa extension, and does using an agent change this timeframe?

most Immigration offices allow the application to the 1-year extension of the stay permit up from 30 days before the expiry of the 90-days stay permit you got stamped when you entered Thailand. Some Immigrations like Bangkok, Jomtien, Chiang Mai, Khon Kaen and others I don't remember, allow it up from 45 days. If you want to know how "your" Immigration interprets it, you need to visit them and ask and also politely ask them for the list of requirements for the application to the "1-year extension of temporary stay permit"

The timeframe varies by location. Some bigger immigration offices like Bangkok, Chiang Mai, and Phuket allow 45 days before the due date, while the rest are 30 days. Using an agent doesn't typically change the timeframe.