3 days or so. It was for a Saison.A decent amount of I 2oz pulled in at the beginning of the starter but once fermentation kicks in its just about keeping the yeast in suspension again. This is why I pump the air in so it gets air the whole time. It's suppose to give the yeast more sterole reserves or something... Been awhile since I looked into it... Have just been doing it for quite awhile. I think Kai wrote something about it not too long ago.
I have used my aquarium pump to get sterile filtered air into the top of the fermenter, to simulate open fermentation. Not sure if it helped, but it didn't hurt.
Hmm there's an idea... Never thought of that. How long did you leave it on pumping air in?