How to change the duration of a mask

If you add an effect to a mask using compositing options, all you have to do is keyframe the mask opacity.

In this example I added a rectangular Mask to the last word, add Gaussian blur to the layer with the blur set to about 80, added Compositing options to the Gaussian blur set to Mask 1, then added a Mask Opacity keyframe at about one and a half seconds to 0% and another one at about two and a half seconds to 100% to get the Gaussian Blur to animate. 

The result is the same as if I had animated the blur, except that the blur only affects the last word.

I hope this helps.

Duplicate/split the layer, remove/change the mask and effects where you don't need them.

Mylenium