When does one end their personal fast? At sunset? or do they wait until 3 stars come out like at the end of Shabbos?
When does the fast start?
- Communal fasts which do not start at night only begin at Alos HaShachar. 
 S”A 564:1
- When does the Fast end? Any fast which one didn’t complete until Tzeis HaKochovim is considered as though one didn't fast. 
 S”A 562:1
Tzeis HaKochovim is when the stars come out. For the time when Shabbos goes out, we add an extra time called Tosephes Shabbos - a minimum of 4 minutes.