Based on my recent trips, here's how long my postcards took to make it back home (BOS):
From...
Tokyo, Japan - 6 business days
Tallinn, Estonia - 7 business days
Riga, Latvia - 5 business days
Vilnius, Lithuania - 9 business days
Nida, Lithuania - 12 business days
My only international postcard that has never made it home was mailed at Heathrow :-/