How long you can safely run your Boulder City unit in emergency heat mode depends on several things, like what kind of unit you have and how big it is, what kind of fuel it uses, and how severe the winter weather is. If they're typical for the industry, your emergency setting and your actual unit should be very close to each other in function. That said, ten hours, if you're getting really close to the emergency scenario, might be your cap as far as a single day without refueling or turning your unit back to regular mode goes, but that's not an industry figure. If your unit has no fuel source or can only run on electricity, then you're risking burnout or electrical shorts if you run your emergency any longer than that.