YU Torah online has a good summary.
The subject is disputed by the Vilna Gaon and Rabbeinu Tam. The first allows only up till sunset; the second up to when the stars appear.
Mishna Berurah 233:14, limits the leniency to recite Mincha until
tzeit hakochavim. He cites the opinion of P'ri Megadim, Eshel Avraham
233:7, who rules that one cannot actually recite Mincha until tzeit
hakochavim, but rather until Rabbeinu Tam's shekiat hachama which is a
few minutes before tzeit hakochavim. [R. Ovadia Yosef, Yechaveh Da'at
5:22, cites numerous Acharonim who disagree with P'ri Megadim and
maintain that according to Rabbeinu Tam, one may recite Mincha until
tzeit hakochavim.] Mishna Berurah then notes that even those who
normally follow the opinion of Rabbeinu Tam should nevertheless show
deference to the opinion of the Vilna Gaon and recite Mincha before
Nevertheless, Mishna Berurah, Sha'ar HaTziun 233:21, rules that even
according to the Vilna Gaon, there is room for leniency in a pressing
situation. R. Ovadia Yosef, op. cit., notes that although the Vilna
Gaon himself does not allow one to recite Mincha after shekiat
hachama, one can argue that within the opinion that shekiat hachama
occurs at astronomical sunset (i.e. the Vilna Gaon's opinion) one can
still maintain that latest time for Mincha is at tzeit hakochavim.
According to R. Ovadia Yosef, the question of whether evening (for the
purpose of Mncha) begins at shekiat hachama or tzeit hakochavim is not
necessarily connected to the question of whether one follows Rabbeinu
Tam or the Vilna Gaon.