It depends on how you level up the characters or how soon you've obtained those stat-increasing augments. If you feel inclined to train a balanced party of 6, then you might find that it takes quite a bit of time adjusting to the level that's appropriate for each dungeon. With a power trio, however, any of these characters can hardly get KO'ed during your journey unless you revisit areas such as the Barheim Passage early on in the game. Some marks were notable exceptions of course, particularly when you're underlevelled, lacking in powerful magicks and attempting them without a guide. By that I'm referring to White Mousse and the 2nd Gilgamesh encounters.

The length of Yiazmat battle? Considering you can leave the arena at any time, the battle can potentially be broken up into several chunks and resumed whenever one chooses to. Much of the fight is automated anyway, so you can opt to do something else, while taking heed of the characters' status.

There is a 50 min strategy introduced by the Ultimania guide, that might be of use to those looking for a shorter fight. I, personally, have never used this strategy myself, because I regard it as an exploit of the Reverse and Decoy trick. I'd rather have a fair 2 hr fight with 1 berserker and 2 damage-dealers pummelling an impressive dragon with their melee weapons, than watch the poor thing follow your lone character like a dog on a leash without getting any chance of hitting back.