Hey, I gave the game a play. Here are the answers to your questions:
1. 7 days. I had gotten all of the upgrades, so it didn't seem like there would be anything new to do
2. Yes. I'm a sucker for upgrades! Honestly if you just let me keep upgrading I probably would have played longer just to see how high I could get them. The balance between upgrades that create more resources vs upgrades that let you harvest the resources faster is good. I would lean into that. Reminds me of this itch game I played before: Box Box Surprise by Happy Distractions Studio
Also, by the time I maxed out the upgrades, things were really satisfying, where I could just click and the things would break almost instantly. It would be fun to move forward with that with a lot more oysters coming, and upgrades costing more and more. And maybe you need new types of upgrades to battle rival otters or something.
One note on the upgrades, though, I actually missed that there was that option on the first night. You might want to make that clearer, maybe with a text popup or something
3. 5 stars (see above).
4. Not really. I didn't have any preferences for particular upgrades, other than I just knew I wanted them to make the clicking more efficient.
5. See above the note about missing the upgrades on the first night. Otherwise, I didn't encounter any issues or bugs. Nice work!