CDG airport to Montparnasse train station in Paris to St. Malo

Most advice I see is to allow 4 hours between ETA and departure time for a train leaving from a Paris train station. I allow six but I hate to rush and usually want to stop for a lunch before getting on the train.

Per another of your posts, did you not you recently complete a trip to France that included, as I understand it, going through CDG. That should give you a reference point to answer your question, no?

Give yourself 4 to 5 hours, when gambling that the flight to Paris won't be delayed.

I sure wouldn't book nonrefundable train tickets in advance. You never know if their will be issues with your flight causing your plane to be delayed. I guess you should evaluate the savings vs. the potential of having to purchase new tickets. On my recent trip to Spain, we landed in Madrid and immediately took a train to Sevilla. Oddly enough, our plane was quite early and the airport was very empty (7am or so) so we were able to get a much earlier train than I would have anticipated. Yep, the tickets weren't cheap, but by not prepurchasing, we had a whole lot more time to explore Sevilla that day.

If you intend to directly go to Brittany from CDG airport, you would lose time going through Paris Montparnasse. You could take a TGV train from CDG to Rennes, and there take a train to Saint Malo. Then in CDG, it is better to allow for 2 hours between the landing and departure of the train.

The previous post is absolutely correct. Avoid a transfer in Paris and take a TGV train direct from CDG to Rennes. Then transfer to a local train to St. Malo (no reservations necessary on the local train). There are a few extremely cheap TGV trains called Ouigo that link Rennes and CDG. I've bought those tickets three months in advance--they are usually only 15-20 euros, so it's not a huge loss if my plane is late. I'm not sure that two hours is enough time however, between landing at CDG and catching your train. I'd give at least a three-hour cushion to be safe.