When did Lot's wife turn to salt?

Prophet Lot's wife was turned into salt stone as mentioned in The Bible. The three Angels sent by God instructed Prophet Lot to leave the city of Sodom and Gomorrah as it was going to be destroyed because of sexual excesses and homosexuality practised in the city. The Angels instructed them to leave the city and not to look back whatever happens. The city was turned upside down and it rained fire and brimstone, but Prophet Lut's wife was sinful and she looked back and was turned into a statue.