How much fragrance or essential oil used in the recipe plays a big role in scent retention. For a strong scent, with regulations taken into account, you can add 50gm of fragrance or essential oil per kilogram of cold process soap, In other words you can add 5% oil(by weight) of cold process soap. For melt and pour, you can add 15gm ounces of essential oil per kilogram (1.5% by Weight), or 30gm (3% by Weight)ounces of fragrance oil. This number will vary based on the scent selection, you may want to reduce fragrance amount for stronger scents like peppermint. This suggested amounts are based on safety guidelines created by IFRA (International Fragrance Association) and RIFM (Research Institute for Fragrance Materials).
Some soapers like to use clay (kaolin, bentonite or fuller's earth) in their soap to help hold scent.
Temperature is another factor to consider when soaping. Some people believe if a scent is heated to the flash point temperature in recipes, it can cause the fragrance to fade.
To ensure the scent stays strong during the cure, store it in a cool, dry place that’s not in direct sunlight. You can also dip a cotton ball or balled up paper towel in the fragrance or essential oil and place it by the soap while it’s curing. That is thought to help the soap “absorb” some of the scent as it cures. Just remember that fragrance oils and plastic do not get along, so be sure your cotton ball is not sitting on a plastic surface.