You can generally use vitamin C serum once a day to get good benefits, especially if you are just starting. Many people use it in the morning to protect skin from daily environmental damage, and some use it twice a day—for example, morning and night—to maximize results. However, if you have sensitive skin, it’s best to start slowly by using it every other day or just a few times a week until your skin builds tolerance. Pay attention to how your skin reacts and reduce frequency if you notice irritation. Daily use of vitamin C serum is safe and effective for most skin types, and consistent use over several months leads to better skin texture, brightness, and reduced signs of aging. It’s important to always follow vitamin C application with sunscreen during the day since vitamin C can increase your skin's sun sensitivity. In summary:

• Start with once daily, preferably in the morning.
• You can increase to twice daily if your skin tolerates it well.
• Sensitive skin: begin with every other day.
• Always apply sunscreen during the day after vitamin C serum.

This approach helps you safely enjoy vitamin C's antioxidant protection, skin brightening, and anti-aging benefits.