"(caturanga Dandasana) ...Remain in this stithi for at least ten minutes..."
"(Urdhvamukhasvanasana)...make the effort to practice until it becomes possible to stay in this asana for fifteen minutes."
"(Ardhomukhasvanasana)... As a result of the strength of practice, one learns to hold this posture for fifteen minutes."
"(Trikonasana)... This asana must be practiced for a minimum of ten minutes. However slowly and patiently we practice this this, there is that much corresponding benefit."
"(Mayurasana)...This asana stithi should be held from 1 minute to 3 hours according to the practitioner's capability"
Quotes from Krishnamacharya's Yoga Makaranda pages 65-69, 97 and 146. See HERE for free download.