Eurostar and Paddington

Which airport? You aren't planning on flying home from London the same day you arrive from Paris are you?

How much time have you got?

Short answer, leave 60 - 90 minutes whichever way and whichever airport. Plus check in time. Plus fudge time - things happen.

Are you planning to go from Paddington to LHR?

St. Pancros is 21 minutes via underground from Paddington. Will you have luggage? Are you up against a time window between Eurostar and your flight? If so, don't cut it too thin.

If you need to get from St. Pancros to LHR, you could splurge and hire a car. It costs ~$85.00 and takes about 45 minutes.

Or you could just catch the Piccadilly Line from Kings Cross (across the street from St.Pancras) and take it directly to LHR. About 60-75 minutes.

Again, which airport?
Do you need really need go to Paddington, or do you just think that is the best way to get to the airport?
If you want to get from St Pancras station (which is where the Eurostar trains arrive), to Heathrow, it is simpler and cheaper to use the tube (Piccadilly Line) direct from St Pancras. take ~1 hour.

St Pancras to Paddington ~20-25 minutes.
Then 5-10 minutes walk from the tube to the Heathrow Express platform.
Heathrow Express takes 15 minutes, but it only runs every 15 minutes so there may be a wait.
Total, nearly as long a the tube, plus extra cost and extra luggage schlepping.

Thank you everyone for your answers and suggestions! We decided to return to London on Thursday evening to avoid any hassles and to enjoy one extra day in London. This is our first trip to London and Paris so I will probably be looking for additional advice.
Again, thanks a million!