How long in advance should you apply for a re-entry stamp on a NON-O visa extension?

There is no strict timeframe for applying for a re-entry stamp after getting a NON-O visa extension. You can apply on the same day you receive your extension or even later, as long as it is before your visa or extension expires. Most commenters recommend obtaining it early to avoid issues with unexpected departures. The re-entry permit's validity matches the expiration date of your visa.