A public disclosure becomes prior art as of the date it is made public, with no "countdown clock" in most jurisdictions. For example, if you publish a blog article on July 1 describing your invention X, it can serve as prior art for any patent applications claiming invention X with an effective filing date after July 1.
There are nuances however with prior-filed patent applications. While such applications are not immediately public, once they are published (typically 18 months after filing), they become prior art retroactively, as of their filing date.
Even if a person independently conceives of a patentable idea, they shouldn't be able to get a patent for it if there exists prior art (although it obviously happens sometimes because the patent examination process isn't perfect).
There are nuances however with prior-filed patent applications. While such applications are not immediately public, once they are published (typically 18 months after filing), they become prior art retroactively, as of their filing date.
Even if a person independently conceives of a patentable idea, they shouldn't be able to get a patent for it if there exists prior art (although it obviously happens sometimes because the patent examination process isn't perfect).