How long does it take a rocket to get to Mars?

You can't go to Mars whenever you want. The two orbits move at different rates, and according to NASA, only once every 26 months do they align in a way that allows the most energy-efficient trip. Missing that opportunity means waiting for the next cycle, which is why missions bunch up within a few weeks of each period: the schedule is dictated by celestial mechanics, not by industry.

NASA puts the interplanetary cruise phase, the stretch from leaving Earth's vicinity to the final approach to the planet, at around 200 days. During that time the spacecraft isn't "aimed" at Mars: it follows its own orbit around the Sun, calculated so that when it reaches its farthest point, the planet is right there. Course corrections are small nudges; the bulk of the work was done by the launch vehicle in the first few minutes.

NASA's Perseverance rover launched in the July 2020 window and arrived at Mars about seven months later, on February 18, 2021, after traveling some 497 million kilometers. That distance explains why the clock can't simply be shortened at will: the path isn't a straight line between two planets but a wide arc around the Sun. Going faster means arriving with more speed, which then requires spending propellant to brake or facing a more demanding atmospheric entry, with a heavier heat shield and less usable mass for scientific payload.