I usually do about 5 minutes or less of warm-ups, then maybe some rolls or something like shikko practice, as I did today. Also, I usually do several repetitions of the first technique as a "back stretch" without a fall since I think it helps to ease into moving with a partner without having to hit the mat right away.
However, at the end of class I take another 5 to 10 minutes for stretching. This is different from warm-ups as it is slow and relaxing. Warm-ups are much faster and designed to get the blood moving and the muscles warm. Although many dojos engage in it, I don't see much point in slow, meditative stretching at the beginning of class since that is actually a cool-down, not a warm-up. I save that for the end of class, and I find it makes a significant difference in how I feel the rest of the day. If I don't stretch after class, I get noticeably tight afterward, so whenever I am at a seminar or visiting another dojo, I normally take several minutes after class to stretch on my own. The best time and place for this is right on the mat since the muscles are still warm. Once they have chance to cool down and tighten up, it is much harder to stretch them back out again.