There are many very differing opinions on this matter, but it all boils down to this:
Part 135: go by
1. FAR 135 duty/rest limitations.
2. FAA POI-approved or accepted Ops Manual SOPs concerning the exceeding of 10 hours flying/14 hours duty. (Despite what anyone else says, this is common practice and varies with FSDOs and POIs, for good or bad depending on your viewpoint).
3. Crew judgement concerning fatigue, which is the final say in this matter. (If your employer threatens your employment status over #3, aggressively seek new employment).
Part 91: it's up to you (unless the company has a specific policy), but make sure the rule on Careless/Reckless operation doesn't come back to bite you in the a**. If you have an accident or incident during your 20th duty hour, you're toast!
In any case, your judgement and common sense should be the ultimate arbiter. Any employer who would fire you for excercising good judgement is doing you a favor. Any potential employer who would hold that against you should be avoided like the plague.