The second disc is about 20-40 hours in terms of story. It mostly involves backtracking to a few places and exploring the parts of the world you didn't the first time. So the game is total about 100 hours to beat give or take 10 hours. You could probably finish it even faster than that but you will miss a lot.
The story really picks up once you get to Dharma Temple as well as the game itself since all the job classes start opening up (btw don't be fooled, the Shepard is an awesome class and I highly advise you make one of your characters into one when the job first becomes available, also make sure to unlock the Teen Idol class as well cause it has a great healing ability that costs nothing) at least for me that is. I would recommend to at least try to get one if not two third tier job classes for the final boss. You can ignore the Monster Hearts cause they have an abysmal chance of dropping and only the Tier 3 monsters give you anything worth using.
As for side stuff, Mini-Medals are pretty easy to do and its fine to ignore the town and casinos but one I recommend doing cause frankly it doesn't require much effort on your part are the pageants. They are based purely on your stats and equipment so basically just playing with equipment or just simply leveling and then dropping by and entering can net you some decent stuff without really forcing you to spend hours on it. Still, I would say to at least play around with everything cause VII has a ton of stuff to offer, especially for a non-remake DQ game.