The two-strike rule is a tried and true method for screening women you've just met.
Though you might apply the one-strike rule when you originally email or text. After good rapport and a date are established, then you can apply the two-strike rule.
One exception is if she provides a rain check both times - in that case, just go silent and if she blows up your phone apply the three-strike rule.
If she ever cancels 3 times call her on her bad behavior, tell her it's over and wish her the best. You should really never take her back in that case, even if she begs. At that stage it's ok to literally make her promise to submit to your every whim - in writing.
Once you've been dating a girl it's more driven by how often she initiates (should be 50% of the time and more than once a week) and how quickly she responds (should be the same minute normally).
NOTE: you can be dating 6 months, and the very first time she ever cancels (or worse, forgets) a date, you can know you're on the way out, so be sure to go LC for a significant amount of time should even one strike happen at that stage, especially when you get "that gut feeling".
ALSO don't be afraid to blow her off sometimes. She will forgive you instantly, trust me. Tell her you lost your phone!
Finally: the only exception to every rule is there's an exception to every rule.