I've had no problems with leaving clay on for a while if that helps

I got to sleep every night with a clay mask on and then wash it off in the morning...I'm sure thats about 7-10 hours every night that I leave it on and I've noticed nothing out of the order that would be 'unsafe'

Sure it flakes during the night but nooo biggie. I love the extra 'mini facial detox' I get with the clay. Any lil bit helps in our war on toxins