The Wicca stones in question may be dried in either sunlight for a full twenty four hours, or the item may be wrapped in a dark black cloth to dry. The stones may also be cleaned in water as well. Another method would be bury the stones in a pot covered with dirt. If unsure if water would damage the stone, however do not submerge the item in water if you are unsure Additionally salt water may be used to cleanse the stone.
In terms of length of time required to clean the stone this is up to the individual user of the item.