There are 2 situations for which the question of Pikuach Nefesh eating non-kosher food is relevant:
1. When a person is expecting kosher food but can't wait.
2. When a person is stranded and only Non-Kosher food is available in the foreseeable future.
1. The Shulchan Aruch Orach Chaim 617 Describes a situation of fasting on Yom kippur when a person would be allowed to eat and break the fast because of saving ones life.
S.A O.C 617,3:
כל אדם שהריח מאכל ונשתנו פניו מסוכן הוא אם לא יתנו לו ממנו ומאכילין אותו ממנו - Any person who smells food and his face changes color in desperation to eat that food he is in danger and we feed him.
The Mishna Berura comments that the same would likewise apply to someone who has only non kosher food available (e.g neveila, chazir, chomeitz on pesach etc.) and is expecting for kosher food to turn up but can't wait.
ובלא נשתנו פניו אע"פ שאומר שלבו חלש וצריך לאכול אין מאכילין אותו. [כ"כ מ"א וא"ר ובישועות יעקב מגמגם בזה ועיין בחתם סופר בחלק אה"ע ח"ב סי' פ"ב שדעתו דאף שאנו אין מאכילין אותו מחמת שהוא בחזקת בריא מ"מ החולה עצמו היודע מרת נפשו יוכל ליקח לעצמו ולאכול בין ביוה"כ בין בשאר איסורין כיון שמרגיש בעצמו שיוכל - לבא לידי סכנה]: If his face didn't change coulour we don't feed him. But The Chassam sofer says that even though we don't feed him (because he looks healthy), if he feels endagered he can take food for himself as much as he needs to save himself because a man knows his own pain and can act accordingly This applies whether yom kippur or other forbiddon foods (i.e eating treif, chometz on pesach etc. in a situation of pikuach nefesh).
So its all dependent on the person if he really feels he has to eat because he feels his life is threatened he may eat regardless of other peoples opinions.
(The following siman S.A O.C 618 talks in detail about someone who is unwell from the start, when we are allowed to feed him but it is beyond the scope of this article to quote that entire siman and was also left out of the question).
2. However if one is not expecting any kosher food and only has treif food available for the foreseeable future (e.g in a desert), instead of waiting forever he may eat straight away what he needs to keep him going. This is comparable to Shulchan aruch orach chaim 344 which states that he is allowed to work seven days a week straight when he is in a desert in order to to find drink and food to keep himself alive for that day (he can start doing Melacha to find food and drink as soon as the day begins) when each day is Saphek Shabbos. (if he knew then when shabbos was he would make sure to get enough to drink and eat in 6 days so there woudn't be pikuach nefesh).
S.A O.C 344,1
ההולך בכל יום במדבר ואינו יודע מתי הוא שבת
ואז יעשה מלאכה בכל יום כדי פרנסתו מצומצמת -
one who goes in the desert every day and doesn't know when shabbos is,he can do melacha every day according to what he needs to live for that day only.
The mishna berura states:
כדי פרנסה מותר שנוגע לפקו"נ שאם לא יעשה מלאכה ימות -
To save ones life its permitted to do melacha for up to what he needs and transgress Shabbos (working 7 days in a row is definitely transgressing Shabbos) otherwise he will die.
When stranded and not expecting Kosher food, he eats treif straight away when he needs(as long as its just what he needs to survive),since each bite is keeping him alive longer he is saving his life through that action of eating treif. (once one knows he definitely needs to eat treif, he doesn't have to wait till just before he drops dead, rather start now and only eat as a necessity from now on).