It takes a long time but a lot less human labor, so the cost can probably be brought way down. Don't know if it can be competitive without reusability though, or at least partial reusability of the engines.
We don't really know that 3D printing saves them much time. Rockets are large cylinders, which there are other efficient ways to manufacture. Other companies will 3D print parts of rocket engines. I'm sure Relativity Space does as well, but they probbaly do that separately from the main body of the rocket with some hand assembly after.