Show trail renderer until the trail ends

I have a character that can run normally, and then super fast with a trail renderer. If I just use TrailRenderer.enabled=false when he stops fast running the trail disappears all at once. What I want to do is have the trail finish, but not start again. I guess, what I'm looking for is something that can give me the status of the current trail, so I can tell when it's done. I tried .IsVisible, but that's not it. I know Time sets the length, but is there a property for checking the current length?