Garbage mattes last for the entire duration of an object, as they are linked directly to it. If you want to control a mask indepedently, use the standalone mask system (drag the default mask object from the media browser onto the timeline and treat it like any other clip).
Alternatively, if you have several mask shapes within a single mask/gmatte, and you want one to 'end', simply drag it right off the canvas. By sure to watch the tweening to ensure that it goes straight from it's final position to off-canvas, otherwise it'll just animate smoothly off the frame, which would look a bit funny.
Let us know if you have any probs.