How long does it take from flight arrival time to clear customs and get on the RER train into Paris where I assume one take the metro to Gare D'Austerlitz.
Customs clearance time at CDG airport depends on the size of your plane and where you are sitting in it. If you're the first to depart the plane at CDG you will pass through customs in less than a minute as each plane or gate area at CDG Airport gets its own border immigration officers in small satellite control points instead of gathering all arriving passengers to a large immigration check area. If you're the last person in a 300 person plane, expect perhaps 15-20 minutes.
Walking to the RER B train station at CDG depends on which gate you arrive at within CDG and which Terminal. Terminal 3 and Terminal 1 are smaller and take between 3-8 minutes to walk to the RER B train station. Terminal 2 is quite long and narrow and could take 12-15 minutes to walk to the RER B train station located between Terminals D and E in Terminal 2.
Once aboard the RER B train from CDG to Paris, expect about 35 minutes of travel time into Gare du Nord station. From Gare du Nord transfer to Metro 5 direction Place d'Italie until Gare d'Austerliz about 15-20 minutes of travel time including transfer walking time.