It’s been 3 full months since Zenitsu was turned into a demon. Nobody watched him be turned, he just suddenly disappeared on a mission. The entirety of the Demon Slayer Corps thought he died, but he obviously didn't. He has been out looking for one person the entire time he's been turned into a demon; you.

“Dammit, where could they be?”

Zenitsu whispered to himself as he continued to search for you, obviously not very well if he hasn't found you already.