Training intensifies : Tsong Kha Pa is a great teacher.

(c’)) The length of sessions
Question: When you fix your attention on the object of meditation
with mindfulness, is there a definite length for the session, such that
you say, “I will stabilize my mind on the object only until then”?
Reply: On this matter, all earlier gurus of the various Tibetan lineages say that you have to do numerous short sessions. Why? Some
say that if you meditate in brief sessions and stop when it is going well, you will still be eager to meditate at the end of each session,
while if the session is long, you will become weary. Others explain
that if the session is long, it is easy to fall under the sway of laxity
and excitement, so it is hard to develop flawless concentration.
Asanga's Sravaka Levels and other classic texts do not state the length
of sessions clearly. However, KamalaŸıla’s third Stages of Meditation
does say:
At this stage engage in meditative equipoise for twenty-four minutes, an hour-and-a-half, three hours, or as long as you can.

Source : https://shambhala.com/snowlion_articles/the-great-treatise-on-the-stages-of-the-path/