Arguably the best way to get to the airport is by taking the RER B train since it isn't affected by traffic in the way that taxis can be (particularly at rush hour when you will be travelling).
The most convenient stop for the 11th arrondissement would be Châtelet - Les Halles which is a major hub. You could either take the Métro or a taxi to Châtelet - Les Halles.
The journey time from Châtelet - Les Halles is around 35 minutes on the express train (which skips some minor stations) and around 45 minutes on the normal train, but I would usually allow an hour for the journey. The ticket costs €10.30 one way.
The RATP website includes a good journey planner.
A taxi from the 11th arrondissement would probably take around an hour depending on traffic but will cost around 5 times the price.