Is a Ghatika an ideal minimum meditation duration?
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.