Is a Ghatika an ideal minimum meditation duration?

I've been reading books written by, or associated with Alan Wallace, that describe a period of time that is new to me - Ghatika (24 minute period). > A session of twenty-four minutes is a good starting interval; for most people, it is neither too short nor too long ... and this is the session duration that the eighth-century Indian Buddhist contemplative Kamalashila recommended for begining meditators. *(Minding Closely: The Four Applications of Mindfulness By B. Alan Wallace p.33)* Wallace goes on to claim that in the Vajrayana tradition a Ghatika is also considered an ideal meditation time because it is the time it takes for the subtle energies to do a full body circuit. Longer duration sits will of course be multiples of a Ghatika. Some poking around on the internet reveals it is a Vedic measure of time - Vedic calculations of time and creation and was measured using a Ghatika Yantra, an ancient Indian water clock . I can find no reference to meditation apart from Alan Wallace references. Can anyone else perhaps shed more light on this?