How much time do I need to take a train from CDG to Bordeaux Saint-John?

Even though that is going to be a more expensive ticket due to the distance, I sure wouldn't prepurchase the train ticket. Planes can be delayed, cancelled or late and/or lines can be long. You also can have really good luck and arrive to the train early. We just went to Spain, flew into Madrid and immediately took a train to Sevilla which is a long train ride. We actually arrived early to Madrid and then were able to easily get to the train station. We got to Sevilla way earlier than I ever would have expected. If I had prepurchased that ticket, I would likely have lost a couple hours sitting in a train station that day. Unless you would enjoy an evening in Paris, I would just plan on buying the ticket when you get to CDG and thru passport control.

I guess that means if you pre-purchase a train ticket in the States and then on the day you arrive in France (Paris), you miss that particular train--then, does that mean your ticket is completely void and you have to pay for a new ticket on a different (later) train?

It's just that the trains a day before cost about 4x as much (E200 one day) as if you booked way in advance!

My plane arrives in Paris at 8:10 am on a Tuesday.

What Tuesday specifically because I am finding a lot more trains than you mention? From CDG there are typically 5 or 6 trains to Bordeaux after the 11h18 that you show. There are even more choices from Montparnasse and the Bus Direct will take you from CDG to Montparnasse for about 18€.

If you purchased tickets inexpensively from CDG, for example under 30€ to 40€, I would feel comfortable with a 3-hour window from scheduled landing to train departure, particularly if you are arriving terminal 2.

You might want to check train schedules again using one of these search engines (be careful about bus options):

Trainline

SNCF

LoCo2

Actually April 9 next year. Tickets are not on sale for the TGV yet. CDG is huge, each gate is actually a separate terminal.

Getting through passport control, waiting for and then picking up one's luggage, and then finding and going to the SNCF TGV platform sounds very time-consuming, if not hectic. But at least I'm on a direct flight from the U.S., and that flight is daily and seems pretty reliable, from past experience. (When I've traveled to Paris, it hasn't been there hard finding the, I think, "B" SNCF line into the city itself as well as buying the ticket itself. It's just everything before that was difficult, as I recall).

OH, I was looking at the TGV trains, not the ones with two "correspondances," which take twice the time to get down there.

You should evaluate the savings by purchasing the discounted tickets in advance and the potential loss that you might incur if you miss the train. It could be that the savings on discounted tickets make up for any risk. I would say that's dependent on the amount of savings and is a personal decision

Otherwise, I would have to go into Paris and then catch a train from Montparnasse the next morning.

This is what I'd do. Your plane may arrive at 8:10am or it may arrive hours later, but it is pretty likely to arrive on Tuesday so you can catch the train first thing the next morning. After an international flight I like to keep the stress low, all I have to do is get to the hotel in Paris. Or you can speculatively buy the 40E ticket, knowing that if you miss the train you'll need the walk-up fare to get to Bordeaux. The right decision depends on the person.

How about fly to Bordeaux? You really do not want to spend the night, it's a waste of a whole day.

As for the train: You can buy refundable tickets, they just cost more. Or you can go with your discounted 11:18, having digested the fact that there is a small but real possibility that you will miss the train and have to buy an expensive day-of ticket, either from deGaulle or Montparnasse (which may be quicker).

There is added insurance, but if you miss the train without canceling, I think they add E50 penalty. It's confusing; I read and speak French but I am still not sure of what conditions apply because it's a matter of interpretation and familiarity with their system. But buying a ticket on the day of departure is very expensive, in any case, from playing with the numbers and dates, so it seems.

I'm seeing day-of tickets for about a hundred euros (2nd class). So if it came to that, it would cost about that much more to get to Bordeaux same day, if you missed your cheaper 11 AM train. But, you'd probably make that train.

A flight to Bordeaux would probably cost about the same and get you to Bordeaux quicker (since the train takes 3-1/2 hours). Is Bordeaux your actual destination?

The problem (I think I noted it above) is that there is only 1-1/4 hrs. to change planes in Paris for Bordeaux (Delta 34).

Yes, I need to get clear on what the difference in price between same-day booking and three-months-out would be for the TGV. If miss the one I book, at 11:18 am, I'd have to be prepared to buy a same-day for the TGV 5 hours later and accept the loss.
GV or not.

I'm never taken a TGV before, and only taken the RER in Paris from CDG into the city, so my experience with French trains is very limited. (I did take a train from Gare Montparnasse to Chartres, though, and just buying the ticket using the machines using a credit card was not exactly easy--the transaction didn't go through, time after time. After 15-20 minutes, I did finally get a ticket, though..

Bordeaux is my final destination for the next four days

Why take the train at all? There are flights from CDG to BOR that leave around 12:40 and get to Bordeaux around 2:00, probably faster than the train and may cost around the same or possibly even less. Only downside is having to get from Bordeaux airport to town, which may be a bit longer than arriving by train, but certainly worth checking into.

This is what you get by doing a search on Kayak: I didn't even think of trying to find a connecting flight to Bordeaux other than the one that arrived 1.25 hours after the Delta arrival in Paris. I assumed that what Kayak came up with was the best possible air itinerary or even that there was no other air connection that day, at least with Delta. If it's another airline besides Delta, that complicates things, of course, as there seems to be the not at all small possibility that the second airline would require that I pick up my baggage in Paris and then drag it over to their ticket counter to check it in.