I answered a question like this previously. How to work with Dried Mushrooms?
Cold water - the best way to keep the original flavour.
It will take at least couple of hours.
I would soak them in cold water for about 15 mins
Pour all the water out. Put in fresh water and keep soaking it. I will do this step couple of times. Remember you may retain the water as flavour, but you have to clean the mushroom and make sure the bitterness is gone. Also, I must say the imported Chinese mushroom must be handled carefully as there have been reports their process may contain some chemicals.
After a couple of hours, make sure the mushrooms are soft and take them out of water and drive them thoroughly. You may keep the water if you wish. The mushrooms are good for steamed and stir-fry.
If you are in rush, then you may use hot or warm water. You still have to go through the cleaning process, but the time required will be less.