F! takes about a half hour, so it will be over about 7:00. If you are seated near the back center you should be able to be able to be back out on Sunset Blvd within ten minutes and out of the Park in another ten. If way off to the sides or toward the front it can take an extra 10-20 minutes.
Once out of the Park you have three choices. A lot will depend on the crowds.
You can take a bus that will get you to the Epcot Front Gate. This will take however long it takes to get on the bus and then add about 20 minutes. You may have to wait for more than one bus due to the crowds.
You can take the FriendShip to the International Gateway, which will put you right next to France. However, again it depends on how long you have to wait for a boat and the boat does make three stops (Swolphin, Y&B, Boardwalk) before getting to Epcot. It takes 20-25 minutes for the boat to get from DHS to Epcot.
Last (and probably fastest) is to take the walkway past Swolphin and Boardwalk to International Gateway. Note that if there is a boat at the Boardwalk dock as you are passing the dock, it may be a little faster (and easier) at this point to hop on the boat as the next stop is Epcot.
But I would think that having the reservation at about 8:15 would be most reasonable. And don't worry, they will not push you out at Park closing.