How long do the story quests last? Does it depends of the numbers of battles, enemies defeated, "explore the area" clicks or something similar? Or it's just random?

@Kirya,

The Story Quests typically complete when you finish the zone. Each zone has a specific structure that you advance through and once you reach the end it will complete your Story Quest.

Ok, maybe I didn't explain well. I'll try with an example.
In the first quest you meet a man who lost his wife's bracelet. My question is: you meet him after a specific number of battles, after a specific number of clicks on the "explore the area" button or random? Hope now it's clear...

@Kirya,

Basically, every zone has scripted events and unscripted events (I call these random events). Your actions throughout a zone advance you through the zone. At certain points, you'll be at a scripted event (AKA, the Old Man in Esior's Fields).

For the most part, advancing through a zone is done by battling or making decisions in events. Random events won't advance you through a zone unless you're in a Dungeon.

Does that make sense?