Q: How long does it take to write a 120 GB disk full with random data?
A: 15 hours, 2 minutes and 27 seconds.
Obviously this depends on the machine. For me it was going at just under 8 minutes per GB, others report around 5 minutes per GB. Also this was using /dev/urandom as input for dd, wich is obviously not really random. I don't even want to think about how long it would take using /dev/random.