Shabbat ends after sundown on Saturday night when there are three stars visible. Depending on latitude and longitude, this is usually between 42 and 72 minutes after sundown.
According to Wikipedia,
There are three widely observed practices, all of which have support in the halachic literature:
- Appearance of three medium-sized stars in the sky (sun 7°5′ below the horizon, or 42 minutes after sundown), as in the Talmud. This is normative practice in Conservative Judaism. In Orthodox Judaism, this position is used widely for the end of rabbinical fasts, but less frequently for the end of Shabbat or biblical festivals.
- Appearance of three small stars widely spaced in the sky (sun 8.5°-8.75° below the horizon): common practice in much of Orthodox Judaism 
- “50 minutes after sundown” is actually a variant of this position. Rabbi Moshe Feinstein ruled this way because most people cannot easily calculate when “8.5° after sundown” will occur, and 8.5° takes 50 minutes at its longest, near the summer solstice, at the latitude of much of the United States.
- 72 minutes after sundown (“opinion of Rabbeinu Tam“): equivalent to other definitions of nightfall, and safe according to all opinions. Common practice in Chasidic and other Charedi communities