From just before shekiah (sunset) until tzet hachochavim (the stars coming out) – see Time of Sunset for assistance in determining these times for your area. This interval is generally about 20 minutes in Israel. In many communities outside of Israel, the custom is to wait about an hour; however, women who tend to bleed with the moch can remove it after 20 minutes.
One should avoid leaving the moch in beyond the prescribed time, as this can lead to irritation and bleeding.