How much time to allow Through customs?

Keep your terms straight. Customs is never a problem. A straight walk without a pause through the Green door - Nothing to Declare. Immigration/Passport control is where the hangup can occur. No way to forecast. Totally dependent on the number of officers on duty and the number of international flights arriving around the same time. Generally you should be able to clear immigration within an hour but it can take longer. Personally I would follow the easier advice of at least a three hour window.

We were there in June. We stood in line for an hour and a half at immigration/passport check. We took the Air France bus into Paris. Total time from deboarding the airplane to Paris was 3 hours.

Thank you for your help. From these responses I think a 10:45AM flight arrival
Will work with a 2:00PM train.

The real problem with booking a same day train is for the totally unexpected. I would give myself at least 4 hours from ETA to departure. BUT ETA is not when you arrive and the biggest issues are likely to be the not common but not unheard of major delays. We have nearly missed a connection in the US for an international flight because of mechanical problems at the first airport and fog conditions at the first airport. Today with the post 2001 security we would not have made the connection.

More recently we had a plane come in 5 hours late having had to turn back to Gander to offload a deranged passenger. (we also had a Thalys come in 5 hours late after a suicide on the tracks near Rotterdam). So when you book the same day you have to have a bit of zen about mini catastrophes.

Being late an hour or two though is not uncommon which is why I don't book same day trains. In our last 10 European trips 8 of the flights were late on arrival; the one a couple of weeks ago to Vienna was exactly on time and we had one come in half an hour early to Rome. All of our Paris flights have arrived a bit late. Immigration averages about an hour for us -- can be longer, can be shorter.

If you give yourself 4 hours, you are probably fine, but there is always a chance of the totally unexpected.

Good information. Food for thought. Thank you.

Our overseas flight was 5 hours late this summer. Immigration was a breeze -- no line at all! You just never know. I would leave plenty of time.

We arrived about 10am last week From an overnight flight and we stood in line for maybe three minutes total. I almost fainted. They didn't even ask any questions, just a cursory glance and stamped the passport. We arrived at terminal 2a, so maybe that is a less busy one, or maybe we were extremely lucky. Trying to get thru the terminal to the rer was a task in itself. Construction happening, little moving room because of people with giant suitcases stopping in the middle of the lanes and people with luggage carts and the huge lines of people at the airport counters...im glad we were arriving cdg and not departing...total chaos.