As Ellie and Lloyd have advised you can do it 30 to 45 days before depending on the immigration office you attend. Remember that they don't ink in the extension on that day, usually you would have to return to the office to get it ratified. I may be wrong, I live here on a non O marriage visa extension and have to wait 30 days from application for extension to get it inked in. So I am not sure you can just fly in apply for extension then fly out without having to come back 🔙 n 30 days for confirmation. I may be wrong and more knowledgeable people may advise differently..

Close the door when you leave