Limiting the amount of frames a mask is active I am following the tutorial 

http://www.aryanwisdom.com/action/study/video-editing/blender-tutorial-overlay-a-video-of-a-head-onto-another-video/

I created a mask where an image will be walking through a door. It seems to work but the mask is active beyond the frames that it is used for. I need to keep using key frames to move the mask out of the frame. How do I limit the amount of frames a mask is active. Same for motion tracking as well. 

Thanks
Andy
Add a color mix node, on the unconnected socket set the color to black. By animating the factor for the mix you can control the opacity of the mask.