Kizmet,
Our experience has been the same as the other that have commented...4-10 days with regular mail.
BUT!
We had an absolute nightmare with registered letters and packages. If you're sending anything from the UK to the US do not, under any circumstances use anything but regular mail. There is no Fed Ex services here in the UK for regular folks (though there are some for businesses) that can get packages overseas in a hurry or recorded. Apparently, the postal services here in the UK have no arrangments with the US Postal Serives with registered or speedy delivery arrangments.
When we needed to get very important documents from the UK to me in the States for my visa, we found that the UK Post Office did manage to get the package out of the UK in 24 hours, but when it got to Miami, where all the mail goes through, it just sat there because no one knew quite what to do with it. Instead of speeding things up, it slowed them down unbelievably.
None of the tracking services go beyond the UK borders either. WHen I called to find out where the package was, all they could tell me is that it was last in Miami, and that if it had left there, they wouldn't know about it because they don't track international packages beyond that point. Very annoying. After getting the absolute run-around (and having 19, yes! 19 different numbers for people at the Miami airport) the package magically appeared after having been missing for about 3 weeks!
Not something I ever want to have to go through again, I can tell ya.