How do I adjust cooking time for smaller batch of bread? I've been making this recipe for bread that I really like, but it yields more than I'd like to make at a time. I'm wondering if I were to cut the recipe by half, how would I adjust the cooking time?
Here's the current recipe:

40g fresh yeast
600ml warm water
1kg bread flour
18g salt

You proof it once for 2 hours, shape it, and once again proof for another hour. Then you cook it in a dutch oven in an oven heated to 480º F for 30 minutes and then 450º F for another 30 minutes.
Your best bet is to use a thermometer.  Most bread is done when the internal temperature hits 190F (88c).  Some people will bake longer because they enjoy a more browned crust. If it were me, I would keep your first bake step the same.  Once you reduce the heat, the second step might not take as long.  I might check the temperature at 15 minutes to see where it is.