You can do them for 1 min each, 2 times each way, total of 4 minutes. That way you'll get most of the intensity from 30 secs, and some of the duration of longer sets. 1 min is good =)
Example:
15 mins standard left,right,up,down,out stretches
4x 1 min behind the cheeks
4x 1 min bundled stretches
4 x 1 min __________ stretch
2 x 1 min A-stretch
2 x 1 min newbie x2 stretches (really great ones)
2 x 1 min newbie x1 stretches
Total 33 mins stretching, and you don't get bored doing the same stretches for endless minutes and reps, which I believe is important to keep motivation and focus high. Once you get un-focused your mind floats around and you get a less intense stretch with bad technique etc.