You are supposed to be in the airport two hours before your flight. However, for the most part that is way too much time. It depends on several things. Will you have your boarding passes before you get to the airport? This is a great time saver since you then can skip the airline booth. If you have to get on the line for your airline it will take some time (maybe a long time). At many airports you can check your baggage in (unless you only have carry-ons) at the curbside check in. This saves a lot of time also.
IF you have your boarding pass, no luggage to check (or use the curbside) your only stop will be at security. I would say you would need only about an hour or slightly more for these conditions.
As for your towncar, they have a rule of thumb that they use and will recommend a pick-up time. You are free to ask for a different time. They normally pick you up about 2 hours before your flight.