Albert Barnes Commentary 2 Chronicles 18:2

"And after certain years he went down to Ahab to Samaria. And Ahab killed sheep and oxen for him in abundance, and for the people that were with him, and moved him to go up [with him] to Ramoth-gilead." — 2 Chronicles 18:2 (ASV)

After certain years - This was in Jehoshaphat’s 17th year (1 Kings 22:51), no less than eight years after the marriage.