The fastest I know of from Paris to Dakar is eight days, (this was when an overnight stay in Nouadhibou was obligatory), it was done by three french guys in a Peugeot 504.
The fastest from Rabat to Dakhla is 18-19 hours done by a Mercedes 380 SEL and a Peugeot 605.
A German guy we know claims to have driven from the German/Austrian border to Dakhla in just under three days, but he took four more weeks to get the car to Dakar.
I did Munich-Bamako in eight days (including convoy) but was shattered by the time I got there.
Seeing that the border from Morocco to Mauritania is now open two people should be able to do it in three days, alone in four, but you need a fast four-wheel drive (Range Rover or Grand Cherokee) and you would have to time the tide and the Rosso ferry properly. Alone in six or seven days is not a problem.
But whats the point?