How many days should I spend in Merzouga?

Two nights is ideal (one to arrive, one full desert day), though even a single night is unforgettable.

With two nights you can unpack, do a sunset camel ride, fill a whole day with 4x4 excursions, sandboarding and Khamlia music, and still catch a dune-top sunrise. One night compresses the greatest hits into 18 magical hours.