The product instructions for using charcoal generally varies anywhere from 1 to 4 pieces of charcoal per litre of water. This depends on the size of the item so check the instructions specific to your product. Personally I tend to put in considerably more as charcoal works by binding impurities to its carbon molecules. Having more charcoal makes the process more effective but providing more surface area for absorbing the impurities in the water. Going by the product instructions the immersion time is generally 8 hours. For myself I find that 12 hours overnight is what works best for me. It is important to understand that depending on the quality of the water and the quality of the charcoal the time required will vary. Obviously the longer you wait the better the results will be. I would recommend testing your water at different time intervals to find the ideal time that corresponds to the result you wish to attain.
See the benefits of Charcoal Water Purification