What did Elijah prophesy would not happen for several years?

Answer: Dew or rain

Elijah declared there would be neither dew nor rain except at his word.

“Now Elijah the Tishbite, who was among the settlers of Gilead, said to Ahab, “As surely as the LORD, the God of Israel, lives, before whom I stand, there will be neither dew nor rain in these years except at my word!””— 1 Kings 17:1 (BSB)