When talking about how efficient a given reaction mass engine, such as a rocket, can be the unit is specific impulse in seconds. That unit represents how long a given engine carrying a given propellant can maintain 1g acceleration.

For context, the most powerful chemical rockets peak around 450s-530s. A nuclear rocket of the sort we can build today would be more than twice that value, and super-efficient ion thrusters can have IspS in the tens of thousands of seconds.

But we're talking about an engine with a specific impulse measured in decades, and as far as anyone knows that means having catastrophic amounts of antimatter. I don't think plucky explorers on a one-way trip are going to have access to a small moon's worth of antimatter, and if they did, imagine how many more interesting things they could do with it than fly to nowhere?