Since 2007, I received 53 postcards from the US (via postcrossing). I live in Germany, so this is relevant to your question.
The average travel time was 11 days, roughly 67% arrived within this time frame. The minimum was 3 days, the maximum 61.
Of course, these are not 100% exact numbers - the "send" date is the day the sender got my address, not necessarily the day he mailed the card, and the "received" date is the day I registered the card, not necessarily the day it reached my mailbox.
Still, all those numbers will always be only estimates, and every single postcard can arrive late. In 2016, I mailed three postcards from Scotland to Germany. The first card arrived after about one week, the second one after two weeks, the third one after three weeks. Since I dropped them all into the SAME mailbox at the very same time, I have no clue why the travel time differed so much...