I've seen 27 hours frequently enough in one direction at least for some U.S.-Greek trips, so 20-24 or more hours is well possible from Greece, a country where a part of the population lives on islands.

Making connections on the way from Greece to the USA and clearing CBP at MSP, the trip can still be long and in the time range mentioned by the OP.

Keep in mind that when the OP is using the word "flight", the reference is to a trip with multiple flights rather than just one flight. You know well enough that there is no non-stop scheduled commercial passenger flight service from even ATH to MSP -- never seen it in my decades of flying in and out of MSP. And even just for ATH to MSP via existing connections with CBP clearance done in MSP, 14-17 hours is rather common; but once someone adds a Greek airport other than ATH to the picture, it frequently enough hits the 17-23 hour range for flights inclusive of transit time.