They Go Off
As far as I know, when the "Squat Switch" says "No Load" on the gear, the fans go off. I'm sure that the do not operate when the gear is up and the doors are closed.
Not sure here, but they may stop even earlier, perhaps when TO power is applied. In most cases, when temps so indicate, the fans should continue to operate during routine taxi. It is an AB, so *more than one* event or condition may trigger the on/off for brake cooling fans.
>>my own question<< All A320 series that I've seen have brake fans, Yet I understand that this is a buyer's option. Are these fans standard gear - or are they really an optional, extra cost item? Does anyone know?