When does a private fast (תענית יחיד) begin? At sunset or at dawn? Is it considered a fast like Yom Kippur (24 hr.) or like a half-day fast?
Are there any sources that address this?
Stack Exchange network consists of 181 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.Visit Stack Exchange
Mi Yodeya is a question and answer site for those who base their lives on Jewish law and tradition and anyone interested in learning more. It only takes a minute to sign up.Sign up to join this community
R Jack Abramowitz from the OU brings sources here and here that it goes from dawn to the time three stars come out.
If an individual wants to take a fast upon himself, he must commit to do so the day before at mincha. [...] It makes no difference that he will continue to eat and drink after this until dawn.
On a private fast, one may rinse his mouth with water in the morning.
If one accepted upon himself to fast without any conditions, then he must do so until three stars come out at night
See also here.
Note there are other opinions on the timing of the end of the fast (see Orach Chaim 562), so as always, consult your rabbi before implementing anything you learn here.