That's the problem, the ATF, AFAIK, has never defined/set that time period. They (the ATF) would rather use the nebulous "INTENT" to do what they want to gun owners. That way, the ATF could go to court and say that they had been watching X and they had a record of him building firearms and always selling them after 6 weeks so they he "intended" all along to sell without a license. Could a good lawyer get him off? Probably, but who knows.

I do know that when a FFL transfers a firearm into their private ownership, it is recommended by the ATF that they don't sell it as a private party sale for a year. In PA that would deal with long arms since private sales of long arms don't require the use of an FFL. A FFL could however, transfer it to himself, then 3 months later, transfer it back into his books and dispose of it wih no consequences.

The problem is that the ATF wants to keep these things in a gray area to make it easier to control firearms owners because people can't figure out what the laws actually say and the ATF never gives a black and white answer.

I will say though, that the ATF is in for a big smack down because more lawyers are questioning, and winning, when they fight the definition of what part of an AR is actually the "firearm".