4 hrs at $1 machines (max coin, non-video poker) should get you free room anywhere in Vegas.

One hr at $1s is ~$1,000-2,000 coin-in depending on your speed of play.
One hr at $0.25 is ~$400-600 coin-in.

A few hours at $0.25 machines (or 1 hr at $1s) will get you noticed by marketing departments in today's environment. At $1,000-2,000 coin-in/day ($50-200 theoretical win), you will cover the $15/day it costs for a hotel room. While this is less than what a host might like (e.g. $800-1,000 theoretical/day for a $250-350 comp/day), marketing is trying to fill rooms.

Sometimes the guideline I use is comps are 4-10X the free play you've earned. This obviously depends a lot on whether the cashback rate is 0.10% (Fitzgerald's) or 0.66% (MGM).

...and the comp thresholds will be very different at downtown casinos, where RFB might only be $50/day.