How long would economy shipping from the UK to the USA take?

Everything depends on how long it will remain in US Customs. It might arrive in the US within a week (or less), but remain in Customs for another week or so. There's no way to know. Figure at least two weeks from the date you ordered it until it's delivered. And don't forget the USPS will be closed Thanksgiving day.

Within 7 to 12 days, at a guess.

One to two weeks is reasonable, but could be longer. You just can't tell.