We hope to ride it the other way next year, blast down RN to get there quite quickly, then ride back up RDGA over 4-5 days, just long enough to not feel too rushed, and can squeeze it into a week of work (using the weekend at both ends).
If anyone has 12+ days I would suggest having a couple of days around the Vercors staying somewhere outside of Grenoble for 2-3 nights, and maybe a day or two at the coast to relax off-bike (if that's your sort of thing)
You could also ride it both ways which IMO is a great way to do a trip as passes are a very different ride in opposite directions - plus if you get bad weather on a pass on the way down, you get a second shot of seeing it in good weather the other way, or take in some of the other passes that run in parallel coming the other way - the only "issue" is this may mean moving on almost every single day which some people may find tiresome.
Of all our Alps trips to date the RDGA trip was our favourite overall