One of the most esoteric elements in the Dragon Ball franchise is the Hyperbolic Time Chamber, known as the Room of Spirit & Time in the original Japanese manga and anime. Housed within the Lookout hovering far above Earth's surface, this room is a chamber that seemingly stretches endlessly and bends the conventional rules of time and space.
And while one of the magical rules regarding the Hyperbolic Time Chamber was assumed to limit visitors to only two incursions inside, Vegeta is seen exceeding that limit in Dragon Ball Super, training in the room multiple times. Here is how the Saiyan Prince was able to gain access to the Hyperbolic Time Chamber more than twice, despite the previously established rules.
Introduced toward the end of the original Dragon Ball, Goku trained in the Hyperbolic Time Chamber in preparation for the 23rd World Martial Arts Tournament. Due to the chamber possessing a denser gravity, extreme solitude and wildly fluctuating temperatures, Goku could only endure its conditions for several hours, lasting approximately one relative month inside the chamber.
During Dragon Ball Z, Goku used his second opportunity to train in the chamber after the arrival of Cell, helping Gohan achieve the Super Saiyan and Super Saiyan 2 transformations, with their one day in the chamber lasting a relative year inside. Vegeta and Future Trunks used the chamber twice, once ahead of confronting Cell and again after the android achieved his perfect form.
Then in Super, Vegeta uses the Hyperbolic Time Chamber three additional times -- with even Goku getting a third opportunity inside the room -- across the course of the series. The Saiyan Prince utilizes the chamber in preparation for the Tournament of Destruction to take on the champions of Universe 6, to battle the alternate timeline villains Goku Black and Future Zamasu and, for the third time in Super, to prepare for the Tournament of Power against the greatest warriors of the multiverse. While there is no explicit rationale given for why Vegeta is able to visit the Hyperbolic Time Chamber so often, the answer is provided at the end of DBZ.
During Dragon Ball Z's climactic Majin Buu Saga, Piccolo decided to lure Buu into the Hyperbolic Time Chamber to fight Gotenks. He reasoned that if the fused Super Saiyan was unable to defeat the villain, he could destroy the exit and seal Buu inside the pocket dimension anyway. While the exit was indeed destroyed, Buu, Gotenks and Piccolo managed to escape, but the Chamber -- along with the entire Lookout -- was subsequently leveled by Buu as the fight continued.
After the Earth was restored and Buu was defeated, Dende rebuilt the Lookout and the Hyperbolic Time Chamber, quietly removing the limit of visits possible. Later in Dragon Ball Super, Dende would again repair the Chamber when Vegeta's training during Super severely damaged both it and the Lookout.
Just as Dende removed many of the limitations set on Earth's Dragon Balls after succeeding Kami as the new Guardian of Earth, the young Namekian also removed the original restrictions set on the Hyperbolic Time Chamber when given the opportunity to rebuild it from scratch. Vegeta and Goku have both since taken advantage, significantly boosting their power levels in a pinch by getting in years' worth of training in a matter of days. And with new adventures for the Z Fighters on the horizon, a return trip to the Hyperbolic Time Chamber may be in the cards.