With breath holding there are different aspects that you could look at, and they all change at different speeds to varying degrees. The good news is that the gains that you loose the fastest, are also the things that come back the fastest.
Just for reference, from fastest to slowest changing;
- Physical CO2 tolerance: Blood bi-carbonate changes how quickly our CO2/acidity will reach uncomfortable levels. It only takes a few days for our Kidneys to filter the blood and return it back to normal, but a few days of training/diving or 1-2 super high intensity workouts can bring it back to where it was.
- Relaxation (more as a new diver) sometimes a few days to a week out of the water is enough make you a little more nervous that you will have lost some training effect, like in other sports you might say you are rusty. But one or two sessions later you will realize that you can still do exactly what you were doing before and you'll be as relaxed as you were again. this problem resolves itself with experience, the more you dive the longer you can go before you imagine that you are "rusty".
- Technique.. If you don't fin for a while it will get a little sloppy but thats nothing 4-5 DYN sessions can't fix.
- Pressure tolerance: For me it takes about 1 month to loose my tolerance to pressure. If I dive regularly, take a month off I can still dive pretty much as deep as I was before with no sensations of pressure.. At 2 months it would take me a few sessions adapting and more than 3 month I basically have to start from scratch taking a few weeks to loose the sensation of pressure past 25m. This gets better after more years of deep diving and is probably different for everyone.
- O2 tolerance/dive response: It takes a very long time to condition your body to save O2, store more O2, and tolerate lower levels of O2. Once you have it though, it will take a very long time to completely loose it or even decrease. Even after 6 months out of the pool/no training I was able to match PBs in both DYN and STA after a few sessions to regain CO2 tolerance and work a bit of technique.
- Mental CO2 tolerance (ability to withstand contractions). Once you can hold contractions for x time, you can pretty much always hold them for x time. Since they are just uncomfortable if you've done it once, you can do it again..