The frequency of applying lip serum depends on the specific product and your individual needs. However, generally, it's recommended to apply lip serum twice a day - once in the morning and once at night.

In the morning, it acts as a protective barrier against environmental factors like wind, cold, and sun. It also prepares your lips for any lip products you might apply afterward, such as lipstick or lip gloss.

At night, the lip serum works to repair and rejuvenate your lips while you sleep. This is when your skin's regeneration process is at its peak, so the active ingredients in the serum can work their magic more effectively.

Remember, consistency is key when it comes to skincare. Regular application will help maintain the health and beauty of your lips. However, if your lips are extremely dry or chapped, you might need to apply the serum more frequently until they heal. Always follow the instructions on the product packaging and consult with a dermatologist if you have any concerns.