It took us a grand total of 75 hours to do absolutely everything in Final Fantasy 16. That includes the main story, all of the side quests, and every hunt. In that time we also obtained the best possible equipment.
If you focus entirely on the main story, you can beat Final Fantasy 16 in around 30 hours, just as Square Enix said prior to release. However, sticking solely to the campaign means that you'll miss out on useful upgrades for Clive, and you won't rack up enough experience points to reach the max level. You also won't accumulate enough ability points to unlock most of Clive's Eikon skills.
This means that actually progressing through the main story can be much more difficult towards the end, when boss enemies become much more powerful. As such, completing some optional activities is recommended if only to keep Clive's level topped up. Hunts in particular are a great way to earn a lot of experience points and rare materials for crafting equipment.