Three days is the perfect amount of time to allocate for your Cusco itinerary, taking in the sights, browsing the markets, roaming the cobblestone streets, and stuffing your face with delicious Peruvian food. Seriously, there are so many good restaurants. Not to mention all of the amazing places to see in the Sacred Valley. If you're a history lover or want to do some trekking in the area, then you could easily fill a few days extra, but overall, three days is perfect.