How to effectively use quick battles (autobattles)?

but if a quick battle lasts longer than 30 rounds, instead of it, a battle calculation is launched, as for AI vs. AI.

a quick battle is identical to a regular battle.

How do these two things relate to each other? A regular (presumably, auto-)battle with fewer than 30 rounds changes to "AI vs. AI"?
And how is the battle duration calculated without actually running the battle?