My personal policy on these assignments used to be that you get 4 hours free, and you agree to be billed for the rest at my consulting rate. Now I am down to 2 hours. Most of my network has a 1.5 hour test as a formality, and it's not that much longer than a phone interview.

Someone I spoke to recently had the audacity to ask for 20 hours of free work in the intro call, and I just shook my head and hung up.