Probably a few hours a week.
I wish there were more things to do besides finishing up shrines and korok seeds. I'd love to see some post game side quests or super bosses appear on the map after you defeat Ganon.
I really like how FFXV has timed quests. They put out specific super bosses or horses of powerful enemies with great rewards and rare materials and weapons.
Example, "Lynels Galore"
Quest details: Defeat the three different colored Lynels near the Akkala Tech Lab.
Rewards: 5x Diamonds, Attack Up Guardian Sword, Ancient Arrows x15, 3,000 rupees.
Selecting the quest in your adventure log triggers them spawning near the tech lab. It would be a cool way to keep the game refreshing and somewhat challenging. You could even give the enemies attack buffs.