How long does a 15 ml bottle typically last with regular use?

A 15 ml airless pump dispenses roughly 0.15–0.20 ml per full stroke. Using half a pump per eye (about 0.15 ml total) twice daily equals ~0.3 ml per day, giving approximately 50 days per bottle. Many users find a single pump (split between both eyes) sufficient once daily, extending the bottle to 3–4 months. The airless design minimizes oxidation of the vitamin C, so the serum remains effective until the last drop if stored properly — keep the cap on, store at room temperature away from direct sunlight, and avoid leaving it in a hot bathroom or car. If the serum develops a dark orange-brown tint or a sharp, oxidized smell, the vitamin C has degraded; at that point the hydrating hyaluronic acid and rose water still work, but the antioxidant benefit is reduced. Write the opening date on the bottom with a marker to track freshness.