Per the RATP site, 46 minutes for the regular train and a whopping 4 minutes less for the express.

(That, of course, is assuming there are no strikes, no people jumping on the tracks, no mechanical incidents...)

The trains generally run about 8 minutes apart.

Don't forget you'll need time to buy tickets... if there's a line, it could take a while.

With that kind of timing, you won't have an "authentic Frenchy brunch experience" (the French don't really do brunch, anyway, except for a few spendy ones in some of the nicer hotels) but you might have a few minutes to gulp down some coffee and a croissant.