I guess it depends on where you position the sulfur cup. I have mine suspended above and not touching the heating element. That way, it takes at least 30 minutes for the sulfur to melt, and start to fill the air.
I don't really know, I stay out of there when it's on, I just leave it for 3-4 hours, and make sure the cup doesn't go empty. Also, an oven glove is a handy thing.
HTH