Not more than 2 hours.
Actually recitation of Gita was in form of plain Q&A where Arjuna is clearing his doubts one by one. Arjuna was gifted with concentration abilities (e.g. aiming bird's/fish's eye or his understanding of other martial arts). Whatever Krishna told as Gita, he was able to grasp with similar swiftness based on his then knowledge of society & scriptures. He could have been quite clear about certain concepts like Swa-Dharma, Paap-Punya, Varnashram.
In today's time, many things are no more trivial. We require detailed explanations to understand Gita better. Hence we see purports in TV or various commentaries online.
Many slokas can be omitted because they were just descriptions:
- ~30 slokas of 1st chapter are description of the war field
- There are many slokas which are used by Sanajaya to describe
Krishna's universal form or situation of Arjuna
- We can omit the texts such as "Krishna/Arjuna/Sanjaya said"
- The conversation was mostly done by Krishna and due to uninterrupted
speaking the speed will be naturally higher and flawless for him
Let's assume that there are 650 slokas worth of text which had to be recited either by Krishna & Arjuna. A sloka recited with a moderate pace takes average 10 seconds from a fluent Sanskrit speaker.
650 x 10 seconds = 6500 seconds = 108 minutes
What were the armies doing?
Many people claim that the time of battle ground was frozen or slowed down by Krishna during the discourse of Gita. But that's a flawed notion, because Sanjaya, Dhritarashtra & VyAsa heard Gita in their real time.
According to this reference, Dhritarashtra didn't hear Gita in real time. Rather Sanjaya would have told him later on about those events due to his capabilities to see past/present/future through Divya Drishti. However, still there are many reasons which are against the notion of time-pause:
- Assume that Krishna really slowed time to almost pause to tell Gita
to Arjuna. In such case, their actions & talks would have looked like
a "fast forward" events to the observers. Possible?
- Assume that the "fast forward" events really happened. In such case
along with others, Sanjaya could have very well missed those. Because
there is no mention that he could also see time-defying events.
- Assume that with divine vision, Sanajaya really saw the "fast
forward" events. In such case, it seems unlikely that he could have
described to Dhritarashtra in understandable way. Because he was
describing as happening in front of his eyes.
- Assume that Sanjaya also got ability to describe them as it is. But
then we have to also see, why should Krishna fast forward the events?
When time required to answer Arjuna's queries itself is unspecified.
What if Arjuna would have understood everything by chapter-2
then (15 mins), will Krishna have to pause the time?
Personally, I don't see a need of making the time still for < 2 hours event.
One may ask, why armies din't start fighting even ShankhnAds were done?
Here are the reasons:
- This was the 1st day of war and the rules were set about the battle
by Bhishma the righteous, so no question of hurling weapons at an
- The war was offered by PAndavas & they relied mainly on Arjuna's
ability. Since Arjuna was on the middle of the battlefield, they
have to simply wait until he comes back.
- Kauravas might be thinking that after seeing their vast army from
close, Arjuna is fearing and arguing with Krishna to back off.
Which would be fruitful for everyone (a hypothetical possibility)
Going back to own camps may not be practical because no one knew when Arjuna's doubts will be cleared. So we can safely assume that both armies waited for that much time. PAndavas in hope of Arjuna coming back and Kauravas in hope of Arjuna backing off.
 Then the Kurus, the Pandavas, and the Somakas made certain
covenants, and settled the rules, O bull of Bharata's race, regarding
the different kinds of combat. Persons equally circumstanced must
encounter each other, fighting fairly. And if having fought fairly the
combatants withdraw (without fear of molestation), even that would be
gratifying to us. Those who engaged in contests of words should be
fought against with words. Those that left the ranks should never be
slain. A car-warrior should have a car-warrior for his antagonist;
he on the neck of an elephant should have a similar combatant for his
foe; a horse should be met by a horse, and a foot-soldier, O Bharata;
should be met by a foot-soldier. Guided by considerations of fitness,
willingness, daring and might, one should strike another, giving
notice. No one should strike another that is unprepared or
panic-struck. One engaged with another, one seeking quarter, one
retreating, one whose weapon is rendered unfit, uncased in mail,
should never be struck. Car-drivers, animals (yoked to cars or
carrying weapons) men engaged in the transport of weapons, players on
drums and blowers of conches should never be struck.