There are many opinions about how tzeit hakochavim (nightfall) is calculated, including 18 minutes after sunset, 24 minutes, 36 minutes, 42 minutes, and 72 minutes.

In Israel, most opinions hold that the moch dachuk should be in place for approximately 18 minutes after sunset. Outside of Israel, according to many opinions, this period is closer to 35/40 minutes.

You need not keep the moch in for 72 minutes after sunset. If the moch is uncomfortable, you may remove it after about 18 minutes.

Rulings on this question vary based both on custom and on latitude (the closer you are to the equator, the shorter the interval between sunset and nightfall). Therefore, it is a good idea to ask a local rabbi, yoetzet, or kallah teacher about the accepted practice in your community.

For assistance with calculating halachic times for any location, see Time of Sunrise/Sunset.