Better obviously to finish before the zman. But there are those that are lenient [Rav Ovadia, Aruch Hashulchan] that it is enough to start before the zman.
In a case of need, one can daven Mincha until 58.5 minutes after shkia, according to Rabbeinu Tam and those who hold like him. [According to them, this would be allowed l’chatchila.]