Certain Kashrut issues require (or may require according to some opinions) that a Shomer Shabbat Jew (not just any Jew) supervise cooking. For example, see here, and here.
How is the status of Shomer Shabbat formally defined? For example:
1) If Jew generally keeps Shabbat but slips up once in a while (maybe, once a month, or once every few months), is he still Shomer Shabbat or does any violation totally destroy his status?
2) If a non-Shomer Shabbat Jew suddenly decides to begin keeping Shabbat, does he become Shomer Shabbat immediately by the very decision itself (ipso facto) as long as his intent was genuine (e.g. he can immediately begin supervising kashrut and, if he dies, dies as a Shomer Shabbat Jew, even if he repents on Wednesday morning and dies before the arrival of the next Shabbat)? Does he only become Shomer Shabbat after keeping a full Shabbat? Does he have to observe a specific number of Shabbats? Two? five? seven? ten? An entire year? A hundred weeks?
As an analogy, the question could be considered similar to asking when a person who is in the process of quitting smoking officially becomes a non-smoker. For some insurance purposes, that has been defined as a year, but in many cases the social or medical definition may vary. Mark Twain infamously claimed that quitting smoking was so easy that he had done it a thousand times. Is Shomer Shabbat similar in that the exact determination of a person's status depends on social factors, or is there a sharp line in the sand? E.g. maybe Reuven has been faithfully keeping Shabbat for the past five years but is still not considered Shomer Shabbat by his synagogue and Rabbi because he had a forty year reputation as one of the most notorious, blatant, public, unrepentant, and generally in-your-face Shabbat violators in the Tri-State Area, even becoming world famous as the person who turned on and off the lights at the Saturday Morning Pork and Shellfish Cookout Competition and Idol-Worshiping Extravaganza every week for twenty years, and the Rabbi has ruled that he must complete ten whole years of observance to prove his repentance.