I would think it is different for everyone. Some find it easy to walk away. Some want to walk away and have every reason to, but can't bring themselves to do it. Some think the status quo is better than the alternative. Myself, I have come to realize that life is just too damn short to be unhappy. If you don't "know" if you're unhappy or someone has to tell you you're unhappy, seek counseling.
Love is strong yet delicate. It can be broken. To truly love is to understand this. To be in love is to respect this.
The above also goes for respect and trust. If the other side can't be trusted, it's over. If you love them, then there is a chance but it takes serious work by both sides.
Alway remember this: If you are referencing a provider, it's already been over for a long time if you are here asking the question you have. You just did not get the memo on purpose.
It depends on what is going on to make you ask the question.
Some things can be fixed with marriage counselors, some can't.
It also depends on how much time you have invested in the relationship 6 months v. 15 years, etc
For relationships that have gone on for a long time, I recommend keeping a calendar of GOOD days v. BAD days (no real detail - just a general assessment)
Now be honest - if the majority of the day was really good, but a small bitch at the end of the day put you in a bad mood (right before you go to the calendar) it was probably still a good day.
AND vice versa....it was a SHIT day ALL fucking day, then she agreed to blow you for 5 min when you went to bed - that is NOT a good day (it may be a historical one, but...) .
Then after 2-3 months, count up the good v. bad.....that should tell you what to do.