The duration of a stretching session depends upon the number of individual stretching exercises, number of reps and sets. For example, if you programmed 10 stretches, for 3 reps of 30sec, 2 sets then it will take more than 45+ minutes with extra time for brief pauses between reps\sets and also to assume the next position. Whether you need this many exercises, sets, reps depends on goals and individual factors. These include whether you are trying to increase flexibility or prevent loss of past gains or for other reasons.
In case of S&S, halos, GS, bridge\TGUs and hanging covers a fair bit of territory as do the 2 specific cooldown stretches (Z & QLO).