The degradation rate of ascorbic acid, also known as vitamin C, can vary depending on several factors such as temperature, exposure to light, humidity, and storage conditions. However, under proper storage conditions, ascorbic acid can remain stable for a long time.

If the powder form of ascorbic acid is properly stored in a dark place, tightly closed, and kept under room temperature, it can retain its quality for several years, typically three to five years or even longer. It's important to note that the potency of the vitamin C might gradually decrease over time, but it would still be safe to consume. After the expiration date, the efficacy of the vitamin C may reduce, but it doesn't necessarily become useless or harmful.

However, if the plastic bottle is not properly sealed or if the powder is exposed to excessive light or moisture, the degradation process can be accelerated, resulting in a shorter shelf life. It's recommended to store ascorbic acid in a cool, dry, and dark place to maximize its stability and potency.