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?
You find references in:


Tsongkhapa's Middle-Length Lam Rim
Kamalshila's Stages of Meditation
The Abhidharma.


Je Tsongkhapa. Middle-Length Lam Rim:


  Indicating the length of sessions
  
  Is there an established length of
  meditation sessions, specified in terms of “The mind is tied to the
  object and placed for just this long?” The major texts such as Śrāvaka
  Levels do not seem to clearly uphold an established length. However,
  in Stages of Meditation III it says:
  
  
    Like that, gradually, you should sit for twenty-four minutes, one and
    a half hours, three hours, or as long as you can.
  
  
  Apparently this was set forth in the context of the established length
  of meditation sessions for special insight meditation after calm
  abiding has already been accomplished, but in the context of initially
  practicing calm abiding it is evidently similar.


In the Lam Rim bring ba, Philip Quarcoo's footnotes specify:


  Twenty-four minutes is one chu tshod, a unit based on traditional
  Indian time measurement and used in the Abhidharma. It is equivalent
  to one out of 60 parts of a day
  
  One and a half hours, in Tibetan thun phyed, literally “half a night
  watch.”
  
  Three hours, in Tibetan thun gcig, literally “one night
  watch”—common measure for a full meditation session.