"The point of the Ngondro, the preliminary practices, is to remove obscurations and become pure. Therefore, the basic guideline for how to practice and how long to practice, is the extent to which we have purified our obscurations. There is no real guideline other than total purification!
A famous quote sums up the whole reason for these practices: “When obscurations are removed, realization occurs spontaneously”.
The only thing that prevents realization is our obscurations and negative karma, and the preliminary practices remove them. When the mind is totally stripped of obscurations, realization is like a wide-open, clear sky with nothing to obscure it in any way whatsoever."
~ Tulku Urgyen Rinpoche