To reach close to the speed of light (~0.9c?) it'll take about 1 year before turning off the engine, and then another year of deceleration if you're planning to

To reach close to the speed of light (~0.9c?) it'll take about 1 year before turning off the engine, and then another year of deceleration if you're planning to do anything else besides travel around at relativistic speeds. Going to Mars using antimatter with the same 1g acceleration the entire time (reversing half way) will take a few weeks and require a small fraction of a gram of antimatter. Ignoring the significant engineering problems like heat disappation, storage if it's not used instantly, you could get two years worth of thrust from antimatter measured in grams and cross the galaxy in just over a decade, in Earth time the trip would be >100,000 years.