Mearls Happy Fun Hour numerous times when discussing Class design and balancing Spells and powrrs Crawford has casually dropped this here and there as well.
Mearls went into probably too much detail on this point, but in actual playtesting they have consistently found thst 3 Round combats are an outlier, and anything longer is a massive outlier. The consistency of this in play is what made them realize that the math for a Spell with lingering damage effects (say, Flaming Sphere) matched with CR calculations, which underlays the Tasha's Summoning Spells and pet adjustments for Beastmaster and new Pet Subclasses.
Which incidentally means they expect an Adventure Day to be 12-24 Rounds of combat, or 72-144 seconds of violent action per Long Rest, but.spread throughout the day.