We are headed out on Sunday for a 10:00 tour of the royal apts. what time do we need to leave Paris to make it?
Depends on where you are leaving from in Paris, where you need to meet, if you need to get tickets or go through security and what time your check-in is for the tour. The trip from the St Michel RER stop is about 40 minutes. I'd give yourself an hour as you may have to wait 10 minutes for a train (be sure to take the VICK or Chateau Versailles train - there are other destinations that run on that line; and be sure to have a ticket to Versailles). It's a 10 minute regular walk from the station to the front of the palace. Factor those times in to the beginning location and requirements for ticketing and check-in.