Simmer until thickened, about 20 minutes. If the sauce gets too thin add a bit of water.
Note: You can also cover and slow-cook this sauce for a few hours if you wish. It comes out nice and tender. Cook it less for a chili sauce with a little more bite from the beef.
Update: I made this great sauce again over the weekend. Our grocery store was out of premium enchilada sauce, so I used a "lesser" brand. I ended up adding a good amount of hot chile powder to the mix, but it still came out fantastic.