This is a great question and one that comes up a lot, both here and in classes. At least one student asks this in almost every class I teach.
In my humble opinion, the BEST way to handle shorter time periods (from 5 to 20 minutes) is to do no more than 2-3 exercises (movements or postures) per practice period. This can include the opening, "Beginning of the Universe" in all cases, just to get centered. I also recommend to take 5 minutes of bouncing, to "jump-start" the energy flow (this is bouncing with feet flat on the ground, focusing the weight on the balls of the feet).
I think trying to fit the entire set of 8 exercises into a 15 minute or less period is good, but not the best. One or two exercises is best for shorter periods, because it allows one to get more into a meditative mind and a nice slow breathing rhythm.
These latter two are at LEAST as important as the postures and movements.
Hope that helps.