For Casablanca, one day might be sufficient, unless you want to use Casablanca as a base for daytrips to Rabat and El Jadida. Both are worthy destinations.
From Casablanca, take a rental car (4WD recommended, although not mandatory) and drive via Volubilis (Roman archeological site) and Meknes (old royal city, most authentic and least touristy royal city in Morocco) to Fes. Spend 3 to 4 full days in Fes. The old town of Fes is larger and more authentic than Marrakech. Stay in a Riad!
From Fes, drive to Merzouga for a Sahara experience (one night is sufficient). From Merzouga, drive via Dades-Valley to Ait-Ben-Haddou. It is quite a drive, but incredibly scenic. Overnight in Ait-Ben-Haddou. Visit the village next morning.
From there, it is just a short drive to Marrakech. For my taste, two days in Marrakech are sufficient. Marrakech has some fantastic buildings, but the alleys in the old town are paved with modern concrete stones and the city is extremely touristy. After Meknes and Fes, Marrakech will be a shock. Also, stay in a Riad there.
So, in total, 10 days would give you a “best-of-Morocco” experience. If you have two days more, even better (then include day trips to Rabat and El Jadida). If you have two weeks, you can include Essaouira and take two days for the drive from Merzouga to Ait Ben Haddou.
Morocco Expedition Tours 