How much change time in airports

I'm assuming you're flying on a Sky team code share flight (Delta, AirFrance, Alitalia are all SkyTeam partners). CDG is bigger and more subject to flight delays because more congested. One and 1/2 hours would be enough if there are no delays. Rome is smaller, therefore more likely to make it in that timeframe. Said this I wouldn't worry too much. If you miss the connection, because of a flight delay, they'll book you in the next flight to Vienna. Air France has 3 flights from CDG to VIE daily. via FCO you have a better chance of making the flight, since FCO is a smaller airport, however Alitalia has one flight only to Vienna currently (probably more in high season). On the way back you'll probably have to take an early flight out of Vienna to make the connection in CDG or FCO. In my experience changing in FCO is faster than CDG. So far I've never missed any connection in either airport (except for once, due to total cancellation), but in CDG I've had to run not to miss my connection to SFO.

Like so many things in life, the answer is "it depends." I like to ask myself, what are the consequences of missing your connection (and what are the chances of that, based on on-time reliability statistics). Are you traveling on a single ticket, or separate tickets?
If on a single ticket, the airline will accommodate you as soon as they can. If on separate tickets, you might be on your own if you miss your connecting flight. What happens to your onward flight if you miss it? Some airlines will do their best to get you on a later flight, others not so much - if you're trying to connect to a low-cost carrier and miss your flight, you might be shocked to discover you have to buy a new, much more expensive ticket. If you're on a frequent flyer award, you may be completely out of luck (next award flight could be available in a few months). Another factor is, if you miss your connection, what are your options to keep moving towards your destination? Does that airline have many more flights there the same day, or will their next flight be a day (or two) later, even if they do their best? Are there many airlines to choose from in a pinch? Trains? It also depends on how tightly scripted your trip is. Do you have a few days in your arrival city before you move on? IF so, it's less of a concern. Bt if you are planning to move on quickly (leaving for another city? getting on a boat?), it could cause your entire trip to collapse. I'm somewhat less concerned about connection times on my way home. Stuck in Paris unexpectedly for a few extra days on the way back? Cry me a river - my office will have to get along without me a little while longer. Lots of factors to consider. For me, 2-3 hour connection times are optimal - plenty of buffer, and a little time to stretch and refresh before getting back in the tube. Hope that helps.

I've been routed by United a couple of times with connection times of 44 min to 90 min. The flights didn't match up when the first flight was delayed. I would always want 2 hours or more for a cushion. Even that amount of time isn't always enough when flights are delayed.

It seems to me Paris is the much closer option. I'd think it reduces your enroute time by at least an hour or two. Duration is a big deal to me, since flying is the worst part of any trip. An hour seems to be cutting it too close but I think 90 minutes for a eurozone flight is doable - especially if you're packing light.

Thank you everyone. Your advice has been extremely helpful.

After price, I shoot for shortest duration. I made a scheduled twenty-nine minute connection in Houston a couple months ago that included a terminal change and the little train ride. Coming through Atlanta the other night we had seventeen minutes (late arrival, same concourse) we made it but the luggage didn't. Who cares, coming home? I go through Roissy quite a bit. Everybody keeps asking this question, but I'm completely happy with an hour if I'm staying somewhere in T2.