Your flight from the US is an overnight flight, so you arrive the next day. Most often the remainder of that day is recommended for some introductory sightseeing and orientation to your destination, followed by an early bedtime! You can wake up the following morning ready to pack alot into your day!
Consider spending at least one full day (even two or more!) in the main cities, as well as some time in the coutryside. You need to plan travel time between points in your trip - and the landscape and mode of transportation will determine those how long it takes.
Durations of trips can be as short as 7 days, but more often 10-14 days.