Well... it depends.
If you only rely on this method for a few days each month immediately after the end of menses, the chances of it failing are MUCH lower than if you rely on it for a week prior and a week after menses.
That said, wiki claims[1] that the latter approach is >95% effective in perfect use, and 88% effective in typical use.
Compare this to withdrawal[2], which wiki claims is 96%/78% effective respectively.
[1]: https://en.wikipedia.org/wiki/Calendar-based_contraceptive_m...