When you add a Follow Path constraint to the camera and press the animate path button
the movement of the camera is controlled by the paths's evaluation time, which in turn is controlled by a generator modifier. If you select the path and open the Graph editor window you'll see a constant line that determines the movement of the path.
To alter the evalutaion time press N and look for the Modifers tab. You can change the in and out points of your path using the Restrict frame Range options. In this example the camera will start moving at frame 20 and stop at frame 100.
You might need to modify also the y= value and the In value to get a curve like this:
A different way to control the movement on a path is to add the follow path constraint but not actiavting the "animate path" option.
Then on the curve you can create key frames for the evaluation time. Go to the frame where you want the movement to start type 0 as the value for evaluation time and I to create a key frame. Then move to the frame where you want the movement to end and type 100 and add a keyframe.
This time you'll see the keyframes on the curve and dopesheet windows.