Looking at the Finger of Death ability, you can see that it has a graphic duration field which is by default 1 second. There's also a field for a delay of 0.25 seconds but I don't think most people will notice that.
From my own tests, it does look like the lightnings fade over 1 second. The only thing I can't replicate for my Jass-created lightnings is the shakiness of Chain Lightning creates.