For dried shiitakes (black mushrooms), most recipes say to soak in hot water for 20 minutes or so.

Some Asian food mavens say the flavor is better if you soak in cold water for at least an hour, even as much as two. I have found a cold water soak of 1 hour is adequate, though not excessive.

In my opinion, the flavor is better after a 1 hour cold soak - however, if I really needed them fast, I'd still soak 20 minute in hot water.

This I find is rarely necessary, since even starting a recipe without warning, I can start the 'shrooms soaking first thing and squirrel the prep around so they are ready when needed.

Incidentally, for fresh shiitakes, they are usually a bit dry and don't cook really well. A 20 to 30 minute soak in warm water transforms them. They then have an almost creamy texture and are very much better than without the soak.