I’ve been asked to respond to this by another board member. Normally I’d steer clear of such a topic for my general feeling is that it breeds a focus on minutia. However when a student asks then an answer should be forthcoming (unless of course no answer is THE answer <grin>).
The short answer is “it depends”.
On what?? On your definition of yoga and the student intention, the intention for the practice, and the intention for the asana in question.
Most people do not have the slightest idea what the purpose of asana is. They’ve never really thought about it in the context of Yoga and no teacher has ever prompted their introspection in that regard. I cannot tell you what yoga is for you. If it is stretching for you then read a book on stretching and decide whether it is empowering for you to follow the letter of that text. It is not stretching for me. So I’ll provide a bit more from the perspective where I sit.
While asana often has by-products we are so familiar with from pop culture (flexibility, serenity, calm, an reduction in stress, strength, muscle tone, etcetera) it is not about those things.
Asana is a process by which we occupy or entertain our mind in the interest of reducing mental fluctuations for meditation. Please consider the Eight Limbs are eight for a reasons and they are in the order they are in for a reason, though that order may not be a chronological one. If you are mentally engaged by holding poses for 10 seconds, then rock on with your bad self. If your mind wraps snugly around 90 seconds, then hold for a minute and a half.
It simply is an issue that pales when contrasted to other issues. A student who can deeply connect with their heart center in poses held for ten seconds brews a much stronger batch then a student who holds the poses for five minutes with no connection whatsoever. Therefore it is much more fruitful, from a yogic perspective, to consider what you are experiencing in the duration of the pose without becoming preoccupied with the time you are spending in the pose. After all, sipping a latte at starbucks and telling your pals you held sirsasana for ten minutes isn’t yogic at all. Its egoistic and therefore counter to yoga.
This having been said, there can be purpose to duration in poses. This can be the case for therapeutics, restoratives, or in certain series’ like surya namaskar, the tibetan rites and so on. I often work in my own practice using duration for very specific purposes. Likewise when I work with students I will use duration very specifically. But I will not use duration for no reason al all.