As often as the next task is what I was told. When you are preparing a meal and cutting chicken, vegetables, fish, and other items with one knife, you should clean it each time you cut the next item. Use hot water and dish detergent to clean it thoroughly, and rinse with hot water. Use a paper towel to dry before you cut the next item. Repeat for each item you cut to avoid cross contamination. If you do not have dish detergent, use boiling water to clean the knife. Leave it in there for a minute, and then wipe clean.
After you are done cutting all your items, clean it one more time thoroughly, dry it, and let it air dry before sheathing it. This will ensure that contaminants will not go into your food, and the next time you need to cut items, it is ready to go.