Create a new template. Open a blank file, change the units settings and use File>Save as template. Make sure the box is ticked for Use as default if you want this every time.
There are other settings you might want to make for your template. When I created a template for the woodworking projects I generally do I set the camera position to be more suitable for the size of the models I do. I also don’t like the wide field of view so I changed the field of view for the camera. I edited the default style so the background is white, the back face color is a green I’d never use in my models (on some displays the default back face color is difficult to distinguish from the shaded white front face color.) and I set Profiles to 1. I want profile edges displayed but I usually don’t want them to be heavier than other edges. Once the style changes are made, the style gets updated and that style is included in the template.