How long can I extend my Non-O visa in Thailand and when should I apply?

A Non-O visa is valid for 90 days. But that doesn't have anything to do with your extension. When you enter Thailand you will be stamped in for 90 days. That is what you need to extend.

You don't say what kind of non-O visa you applied for. But the only option for applying a non-O visa is the 1-year extension based on the same reason the non-O was issued. It can be applied for when you have 30 days or less left on your entry stamp. Some offices allow 45 days early.

If you are married to a Thai or have a half Thai child, you may also qualify for a 60 day "visiting Thai family" extension one time per visa.