Camera: I used a 600 with a ND filter. The problem with loading Time-Zero film in the 600 is not a real big deal. The 600s have flat springs on the bottom designed to push the film cartridge up. The Time-zero cartridges have opening on the bottom that could become hooked on the spring. I fold a bad 4x6 print and lad it over spring in camera, load the cartridge as normal and put the 4x6 out.
Day (evening) of my first try it was a little over cast. Took 3 or 4 shoot with the ND filter over the lens. The shoots all can out black. Gave up. Next time bright sun and I put the filter over light meter eye. The print where good, used a chop stick to do the manipulation (new not used)
Not sure of the time, I guess less that 5 minutes, think I should let them set longer