There are up to 800 hidden quests.

The time limit is long, but you can reach level 80 quickly. However, from that point on, there are many puzzles, so it takes a bit longer. Ads pop up 4-5 times, which breaks the flow. You might have to give up on a few hidden quests to proceed. Otherwise, you might get stressed. Instead, quests to clear 5 or 10 items always appear. It's good for casually passing the time^^