Erg Chebbi is the Sahara most people picture: a wall of orange sand rising up to 150 meters out of flat stony desert, 28 kilometers long and a few wide, right against the Algerian border. Merzouga is the village on its western edge. Caravans bound for Timbuktu once crossed this region, and now it's where tour minibuses end up.
Be clear-eyed about what you're buying. The camel ride and the tent in the dunes are a tourism product, and half the village earns a commission. The landscape is real and the quiet at night really is something, though. Three days is the right length: one for the dunes and a night in camp, one for the villages and the dry lake around the erg, one for the oasis towns of Rissani and Erfoud. More than three and you're repeating sunsets, fewer and the drive eats the trip.
The drive is the catch. Marrakech is about 550 km and 10 hours away, so these are three days on the ground, with one or two days of road before and after (see Getting there in the tips and the first day below). Day one is the dunes and the camp, day two sunrise, Khamlia and the lake, day three Rissani and Erfoud. Start on a Friday so day three lands on a Sunday, the big Rissani market day. For the cities at the other end of the road see 5 days in Marrakech and 3 days in Fes, and for the country in general Morocco.