How much time should I allow to get to CDG from the 9th arrondissement by taxi

Thats hard to answer as it will depend on time of day and traffic.

To be generous allow about an hour, but it could be 35 minutes.. so as I said.. depends. You do know its set pricing now.. it will cost you 50 euros. Period. Make sure taxi meter says 50..

The nineth, I've found, is actually a convenient spot to get to the airport by car. Usually if I take a shuttle, I'm the first off when I stay there.
I always go 4 hours before my flight because I'm always afraid of missing my flight. It rarely takes an hour to get there if you miss rush hour. Depends on what time you're flight is.

It depends -- what day of the week and what time of day?

Having just finished our Paris trip and done exactly this, I would suggest planning on 1 hour with the likelihood that it will only take 30 minutes.

BJ's advice is good . . . I usually allow an hour. Hopefully it doesn't take that long, but it might.

Be sure to follow your airlines' directive about how long it will take you to check in.
Then - if you are travelling around 6 to 7:30 AM, add 35 minutes to that.
After that, you should add 60 to 90 minutes for rush hour traffic.