For the original question:
We do our ba shi almost every day of class (4 times a week).
we do it for 30~60 seconds per stance so 4-8 minutes total. depends on the day and what else I have planned for class.
Sometimes (maybe once a month) we'll do ma bu for 2-4 minutes w/o doing any of the other stances.
I like to mix stuff up a lot though. The other day I had them do 10 break falls then, 30 seconds in a posture, then another 10 falls, repeated till they did 30 seconds in each stance. They ended up w/ 4 minutes of stance training and 80 falls in about 8 minutes total.
they hated me :D
also, we don't just exercise our legs while doing posture training. Our arms are held in different blocks and strikes while holding the posture so our arms are getting some development as well.