As an individual, I can just by it in the US for personal use, and carry it over. But if you intent to transporting them for selling them, the story is very different.
Legally, you are required to pay VAT and import duty when buying in the US and importing it, even for personal use.
Chances are, you aren't checked. But if you are, having the original packaging or charger will be noticed by customs. They might even have data to determine country of origin from a serial number, or notice that it's a version that wasn't available when you left for the US etc. And if they ask, the burden of proof is yours to show you bought it in the EU.
How long does one need to stay in the US for that not to hold ?
I lived there for one year, and bought a lot of stuff in the US for personal use (like clothes). When I moved back to Europe, I just moved everything with me without paying any VAT.
I was worried that they would make me pay VAT from socks and underwear to laptop, phone, and everything. Asked about it, and was told that I didn't had to pay anything.
This is not EU specific, duty is typically due on consumer good when you move them between countries if you haven't owned them long enough. It's not always enforced or enforceable of course, but it's there. See for example people buying cars on either side of the US and Canada border for use on the other side.
As an individual, I can just by it in the US for personal use, and carry it over. But if you intent to transporting them for selling them, the story is very different.