How long does it take to use opening preparation at top level? A ChessBase report for the ongoing Cairns Cup gives this paragraph:


  The first game to finish was Humpy Koneru versus Ju Wenjun. Out of an English Opening, Humpy quickly found herself out of book with the white pieces. The Indian was on the back foot but managed to keep things under control until a repetition took place starting on move 19. Later on, she explained that it is difficult to face someone who comes from preparing for a world championship match, as their level of preparation is particularly impressive after such an important contest.


The implication is that Ju Wenjun is likely to be very "booked up" for a while after the world championship match, but only for a while, and she'll be easier to face some time in the future.


How long does it take until Ju Wenjun is no longer very "booked up"?
Why doesn't Ju Wenjun stay very booked up for as long as she's playing professionally?
As Brian noted, theory can move on quickly, but that does not remotely mean that everything that was prepared is lost in a short period of time. I think that the biggest issue was just that prepping for the world championship was much more intense, and rigorous, so Ju is going to be that much more prepared, and thus, a much more dangerous opponent than usual for while, but not just because of opening preparation. 

On the other hand, I disagree that just because they have outstanding memories, that they do not forget much...virtually all memories do fade, and it can take a long time to use some prep. There is also the enormous volume that they have to memorize compared to a club player. They certainly do not forget the basics, but it is easy to transpose a couple of moves when you are already deep into the opening. I have read about this happening many times, and in the first round of Tata Steel 2020 just last month, it happened to Vladislav Kovalev in the first round versus Alireza Firouzja, and Kovalev went down without a fight (he played 16...c4? when he needed 16...Nd7 17.Ra3, and only then 17...c4). It can happen to the very best: Here is a tweet from Caruana talking about Magnus forgetting his prep. They memorize SO much that it is still human to forget things that they have not looked at for a while.

It can also take years sometimes to be able to use an opening surprise. I remember reading on at least one occasion, it took Kasparov years to use some prep, and I have heard numerous such stories about top players in my 40 years of tournament chess. Here is a link to a story about Kaspy waiting years, and using some prep against Michael Adams. That game can be found here, and 14.Qh4 was the novelty.

I also think that there is another huge factor that Humpy did not touch upon, and that is that when you play a major round robin (relative to your rating), or in this case, a world championship match, it is so intense, and the level of play is so high, that you usually come out much sharper than normal. I played a couple of round robins against all strong masters, and I felt like I was better than ever when I was done each of them.

Of course, professional players at this level continue to work on their openings, maybe daily, but it is doubtful that it is quite as intense as when she was preparing for the match. In essence, she still stays fairly booked up at all times.