Realistic or no: Plane arrives at CDG at 10:30 am. I catch a flight to Venice at 12:50. Same terminal, different airlines. I need to go thru customs. Will I make it?
Where are you flying from? Do you have to go through Schengen immigration? This is not a connection but two separate flights. Immigration easily takes an hour and can take two; planes are sometimes late. Our last two flights were a bit early but in the preceding 10 flights we were late all but once.
If you are flying from another Schengen country and the flight is on time, then you probably make the flight as you won't have to go through immigrationt. If from the US, Canada, Australia then odds are pretty good you won't but if everything goes smoothly you might. We got through immigration in about 20 minutes this year; last year we were business class and had a fast pass and also got through quickly but the lines for coach passengers were blocks long. If three big planes come in just ahead of you and immigration is poorly staffed, you could easily spent your whole connection time in line.
If you are flying on a budget airline like Easyjet, they close check in 40 minutes or more before the flight, so suddenly you don't have two and a half hours but have lost another 40 minutes.
I would not book that close, but depending on all those other factors you might have a shot but might well not make it.