Just came back from 2 weeks in Europe flying between cities. Everything was basically on time. Unless flights have bad history of being late 3 1/2 hours should be plenty of time.
travel time between airports is about 3o minutes.
http://www.aeroportsdeparis.fr/ADP/en-GB/Passagers/Access-maps-car-parks/Paris-Orly/Access/getting-to-or-leaving-paris-orly-by-taxi/

The RATP site calculates 1h20 to go from one airport to the other with the RER, changing trains once:

http://www.ratp.fr/itineraires/en/ratp/resultat-detaille/start/Aeroport+Charles+De+Gaulle+2+Tgv+%28RER%29%2C+Tremblay-en-France/end/Pont+De+Rungis+Aeroport+D%27Orly+%28RER%29%2C+Thiais/is_date_start/1/date/2012-11-21/time/15%3A30%3A00/route_type/plus_rapide

The route on the link given by grendel (taking the RER B from CDG to Saint-Michel Notre-Dame and the RER C from there to Pont de Rungis) does take 1hr20min. However, you'll need to allow a little more time to take the airport shuttle bus from Pont de Rungis to Orly.

The truly fastest route would be to take the RER B to Antony (60 minutes) and the driverless Orlyval shuttle train (8 minutes) from there to the airport.

The fastest route might actually be in a taxi depending on the time of day.

Taking the RER B to Antony is painless, having only one transfer in a non-crowded station.

I've flown Easyjet a couple times in Europe and they were always very much on time, I have never heard that they have a reputation for being late. In any case, wouldn't count on that.

I would count about 70-80 minutes and absolutely use the RER B.

Merci!!!

Schnauzer

Me again... Kerouac when you say the RER B are you referring to option A or B of TimS's? Can you please clarify, thanks.
schnauzer

me again, again... I have looked at the RER plan and understand that the RER goes from CDG to Antony, but can't understand Michael's comment about changing at a non crowded station. Which station is he referring to? Does he mean Antony to get the Orly shuttle train to the airport itself? Or is there another change before this station. Michael can you please tell me.

Thanks,
Schnauzer

The connection is to be taken at Antony. This station can be considered as not being crowded

You shift over to the OrlyVal at Antony. It is an automatic shuttle that takes you to the airport. 90% of the people who disembark at Antony are going to the airport.

A Paris station, assuming a transfer from RER B to RER C, is usually crowded and can be confusing. For example, at the Gare du Nord we got on the wrong platform because the signs were not clear and I forgot that the French rail system, of which the RER is part, travel on the opposite tracks from most other rail and subway systems. No such error is possible in Antony.

The signs are not clear??? There are screens everywhere informing you of the destination of the trains and all of the stops.

But my answer IS clear, thank you both so much. That is great info.