I thinks it's 7 days of inactivity.
I finally saw the 0% resume problem.
Following a crash, my incompletes did not resume on startup, even after trying all the tricks. This happened a couple times using LW3.2.0j173 on Mac 10.2.6.

I'm guessing the downloads.dat file in the incomplete folder became corrupted.

Did you have a crash or get a pop-up error message in your session before the incomplete wouldn't resume?