CDG is a mess. If it is busy at all going through the check in/security/ passport check can easily take 2+ hours. Even if you hop the express RER into Paris it will still take you 30 minutes to get to Chatelet. So, between coming and going, 3 hours at checkin/security, you are probably looking at 45 minutes to see Les Halles (which isn't that cool... cheap crepes though).