Mix your levain about 20 hours before you want to enjoy it. So before bed if you want to mix dough in morning and bake by dinnertime. In the morning if you want to make your dough to rise overnight, shape and bake by lunch.
Mix dough. Let sit for 20-30 minutes. Knead until it comes together.
Let rise until doubled, which depends on your house temperature but should be about 8 hours.
Divide into two equal parts. Shape, then let rise until double, about 3-4 hours.
Bake at 375° for 20 min, turn loaves around and bake another 20. You know they are done when they sound hollow when tapped, or use a thermometer to check for 190-200°F.
Notes
You can use up to 300 grams of whole wheat when mixing the dough, and the rest all-purpose. Whole wheat is just too heavy to use for the entire amount.
You can also shape these into French bread loaves. Bake for about 25 minutes total.