You can register with Slamstox at any time during the year. Some athletes start the trial in December, others in March.
In general, the sooner you start the process, the better. There are simply more places available in teams, so you have a better chance of your dream place! For example, you may have a better chance of a good scholarship when you start your process when you are in HAVO 4 or VWO 5 than when you start the process when you are in your final exam year. However, this is generally the case, it has also often happened that athletes also end up at top universities within an accelerated process.