As far as conventions go, I check some larger retailers from Europe, like Zalando, and they offer a 100 day return policy [1] by convention.
Maybe Switzerland is the outlier here?
[1] https://en.zalando.de/faq/Returns-and-Refunds/About-our-100-...
By contrast, in the US it all depends on store policy and, apparently, luck. I had some pretty bad customer experiences in the US, and there was nothing I could do then.
For example, this whole Saga on GamersNexus (check other videos) is a good indicator that the US system may not be perfect https://www.youtube.com/watch?v=2fnXsmXzphI
As Steve says in the video, in the US you have no recourse. There's no law. The shop in question didn't feel like being "nice" and the person is out of 500$. For Steve, this amounts to a scam, but the burden of proof is on him. For specifically this reason, the EU has a law that says otherwise.
In practice some stores do accept returns if you have a receipt, but personally I have never done that, didn't know it was even an option before an expat was asking about this online. The default is to sell forward in online second hand marketplaces. Especially gifts you receive, those are never returned (as apparently happens in the US said that expat).
Most instances were in Switzerland as I mentioned, although a few were in the EU. Some weren't defective items but rather unneeded items. Returns based on changing one's mind, even if the item is in unused condition and remains re-sellable, seems to not be a "thing" in at least several European countries whereas American consumers typically expect these consumer protections not by law but by convention.
Another electronic device was defective, and even after selecting this in the online purchase return form I was charged a restocking fee despite returning the item within a few weeks of purchase. Not budging on this and refunding the fee would be unheard of for any serious US retailer.