I wouldn't think anything longer than 2 weeks unless it was delayed by customs.
From the day it's posted it shouldn't take but a day at most to hit London. From London to NYC same day or the next. Estimate 2 days in customs, unloading, sorting, clearing then back into the mail system. From there it goes back onto a plane and sent to Charlotte, Atlanta, Louisville, or maybe Memphis. Add one day for every major city between you and the closest link.