Slowing down gaussian blur

I answered my own question (to a degree) from a couple of posts back. I apologize for posting without playing around with it. So, obviously, I can use iMovie's Gaus Blur to simulate an out-of-focus shot and decide when to end the effect in a clip. Now, however, I need to slow it down. No matter where I end the effect, the scene jumps back into focus pretty quickly. I'd really like it to be very gradual. Any suggestions?