Final Cut Pro - Altering a Filter Range
Filter durations can be quickly modified. Here's how.
If you want to limit the range of a filter AFTER you've applied it, double-click a clip to load it into the Viewer and click the Filters tab.
Grab and drag the two heavy black vertical lines at the top of the filter to alter the range of the clip to which the filter applies. If you don't see the black lines, they will be at the very beginning and end of the clip.