There are many different opinions on this. When you get a coral or snail or whatever that has ich on it, it can take from 2 to 28 days for 99% of ich, under tropical temps (about 78 degrees), to come out of hiding and try to find a fish host. Of course you don't have any fish in there, so it won't find a fish host within 24-48 hours and will die off. To speed up its life cycle, keep the tank around 80 degrees. I do 6 weeks. If you want to be 100% sure, instead of 99.5% sure that the ich is gone, do 72 days then.