My wife and I are going to Morocco for the first time. Most important for us on that trip is to ride the camel and night in the desert under the stars, so for that we booked a night in the desert with a tour operator. Can we drive from Marrakech to Merzouga in one day, or will it take more than one day? The trip to the desert leaves around 16:00.
In recent years the road has been sealed, so it's a lot better. It can be done in one day, according to Wikitravel, and indeed, Google Maps suggests 7 hours 27 minutes.
If you don't want to drive that far or for that long in one day, you could consider a bus transfer which are estimated to take eight hours.
Due to my personal experience, I would avoid driving for such a long stretch in Morocco. If the road quality has improved, the skills of the local drivers have not. I can assure you their driving style is very different that what you might be used to: aggressive and unsafe. Moreover, the fact that you don't know the road also means you will be driving even more defensively. This will add an extra source of stress on you. 7 hours of stress is a very long effort. I would personally take the coaches. Indeed I used Supratours from Marrakech to Essaouira without a glitch.
There are Supratour coaches serving the line Marrakech - Merzouga. These leave at 08:30 and arrive at 20:59, needing approximately 12 hours 30 minutes, according to the ONCF timetable website. Therefore you would have to plan your journey accordingly.
There are also mentions of night coaches, such as this tripadvisor forum post from 2011. I could not find any more information about these though.
Your best bet is to go to the Supratours coach station in Marrakech to ask or more details, seeing as there isn't much available online.