The GR20 takes approximately 14 - 16 days to complete depending on your fitness and the weather. It is likely that you will need a day at the start and a day at the end, to get to and from the appropriate airport.
If you want to really enjoy the trek it is worth spending a bit of time on the planning stage. It's important to select the right gear and maybe book your nightstops.
Although everyone's needs and expectations are different, hopefully you will find some guidance and tips in this site to maximise your success of this great walk.