Stamina training need around 1-2 weeks for a pop. So yes, it's quicker than most of the other trainings.
I can't find the english link at the moment, but this is from the German Help forum. At the top "Kondition" means stamina.
http://www.buzzerbeater.com/community/forum/read.aspx?thread=39570&m=1
We have both said a lot of things that you are going to regret.