Lilian,
Thanks for writing to the Moms Panel!
I assume that the biggest "unknown" of your question is the drive to Downtown Disney. I would say that this typically takes about 30-40 minutes, assuming that you aren't going during rush hour (add approx. 15-20 minutes if you are).
The time to get from the plane, get your luggage, and pick up your car is probably in the range of 45-60 minutes, but it's really no different from almost any other airport (except you have a brief monorail ride, which only adds a few extra minutes). Of course if the luggage is slow or there is a long line for the rental car, the time can be longer. I'd highly recommend trying to check-in for your rental car in advance online, if that option is offered. Also, it can save time to be a member of the car rental company's frequent rental program (e.g., Avis Preferred, Hertz #1 Gold, National Emerald Club) -- most of these programs are free or relatively cheap, and can be big time savers during check-in.
Hope that helps!
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