This is a clue, a definition God gives what the meaning of "eternal fire" is,the Bible interprets it's self.
"IF" it was a "literal fire" then these cities would still be burning today.......


Also, God is govern by his "own" laws. Here he commands punishment is not to continue after a certain point,the punishment,wadges of sin is death.

DEUTERONOMY 25
2 And it shall be, if the wicked man be worthy to be beaten, that the judge shall cause him to lie down, and to be beaten before his face, according to his fault, by a certain number.
3 Forty stripes he may give him, and not exceed: lest, if he should exceed, and beat him above these with many stripes, then thy brother should seem vile unto thee.