UPDATE: Magnus Carlsen announced on July 20th that he wouldn't defend his title in the 2023 championship.
He said he didn't feel enough motivation and wouldn't gain much from it. He said, verbatim, "I simply won't play the match." He didn't rule out the possibility to participate in such matches in the future but added you should not particularly count on it. However, he is going to continue playing FIDE events and doesn't plan to end his career any time soon. He still thinks of reaching 2900 Elo.
You can watch his interview on YouTube to get more details:
kunal chess (channel), video: It’s official! Magnus Carlsen will NOT defend his title, time stamp: 51:03. Before that moment they mainly discuss poker and Magnus's experience in Las Vegas.
OLD ANSWER (July, 9)
I don't think there's a very strict deadline set for Magnus but the director general of FIDE said that there should be some information in about a couple of weeks, around July 20th or so. That's what they expect of him. FIDE can't wait ad infinitum. They have to plan a lot of things ahead. It's probably preliminary information. The director general talked to Magnus and they seemed to have a more or less constructive conversation about Magnus's wishes, which was private, I guess. However, you can find the director general on YouTube. He discusses this issue during chesscom broadcast of the 2022 FIDE Candidates with Robert Hess.
I don't see many uploads with FIDE Director General (Emil Sutosvky) discussing it with Robert Hess. However, chesscom original broadcast of 2022 FIDE Candidates is still available as of now (July 2022). It's round 14, time stamp 1:32 on YouTube.
Note. I've decided to make this answer Community Wiki. I think it's in line with questions and answers of that sort.