A question about a recipe: Steamed Brown Bread

I have a question about step 5 on the recipe "Steamed Brown Bread" from thirschfeld. It says:

"Place the pot onto the heat and bring the water to a boil. Reduce the heat to a simmer then cover the pot with a lid and set a timer for 1 hour. "
I live at high altitude (5280 ft.) so am wondering how to adjust the steaming time to make sure it's done inside...