We have explained that Shabbat begins at sunset and that we are instructed to add some additional time at the beginning of Shabbat. Thus, we always receive Shabbat before sunset (see this).
Now, at what time does Shabbat end?
We must bear in mind three time-references: sunset (sheqi'a); nightfall (ṣet hakokhabim) or "dusk", which Halakhically is determined by the visibility of three medium-size stars, and twilight (ben hashemashot) which is the lapse between sunset and nightfall.
Technically speaking, Shabbat concludes at nightfall. And similar to what we do when Shabbat begins, we extend Shabbat, adding a few minutes after nightfall before we actually end Shabbat.
Now, let me explain briefly why we cannot set a universal time for the end of Shabbat. The conclusion of Shabbat depends on the length of the twilight time-zone. And twilight varies depending on the year's season and especially on the latitude of the observer. In the Ecuador, for example, twilight would last approximately 10 -15 minutes, while in some cities of England, a couple of hours. Another point to bear in mind is that there are different criteria to establish what twilight is (for Rabenu Tam, for example, "sunset" takes place at a later, time!) even in the non Halakhic world. We find that there is "civil twilight", "naval twilight" and "astronomical twilight" (see this), depending on the angle of the sun below the horizon, which obviously affects the darkness of the sky and the visibility of the stars.
In Israel twilight is calculated at 13 and a half minutes. In New York, for example, it is estimated by most Rabbis that twilight lasts for approximately 30 minutes (this is close to the "civil twilight" or "civil dusk"). Therefore, what is beyond debate is that in New York you cannot end Shabbat less than 30 minutes after sunset. As a result, in some communities they would end Shabbat 35 minutes after sunset, extending Shabbat for just 5 additional minutes. In most communities they would end Shabbat 40 or 50 minutes after sunset, i.e., 10 or 20 minutes extension (tosefet). The truth is that, since there is no minimum mandatory length of time that it should be added to Shabbat, the amount of time you add to Shabbat once twilight is over is a matter of conventionalism or Minhag (tradition, custom).
Shabbat Shalom
Candle lighting in NYC: 4:56 pm (sunset is today 5:14 pm)
Shabbat ends in NYC: 5:54 pm (30 minutes twilight + 10 minutes additional time, tosefet)