The DC stretches are done every sessions
Yes, it is meant to be progressed on
You should aim for a stretch of 60-120 secs. On the weigted ones, when you reach 120 secs, increase the weight
Obviously, for some of these stretches (biceps, quads, hamstrings, shoulders) you wont really be able to progress as far as the weight is concerned. In those cases tough, you can play with the tension using your own body.
The DC stretches are done after you are done with a muscle group (not at the very end of the session)