When I say short, I mean 1 week, maybe 2 weeks tops. Few places that want the scammy sort of contract-to-hire are going to bother with someone who expects to be paid their regular wage for a one-to-two-week time period.

Anything longer than 2 weeks, and I'd expect full benefits, possibly a signing bonus, and I would speak at length before hand to get specifics on exactly how I will be evaluated and which criteria will be used for determining if a permanent offer will be made.

This scares away most crappy companies and acts as a good filter to find good employers.