The longevity of the CBD tincture really depends on the base it is in. If it is an oil/glycerin base then it would be a shorter shelf life as oils do go rancid over time. If the CBD tincture is in a 190 proof ethyl alcohol base like our tinctures are, then the shelf life can be 2 years up to even 10 years. The high proof alcohol acts as a preservative if stored from heat and light. I would suspect that the compounds in the tinctures will over time decrease and therefore decrease potency, so if you have had a bottle for over 2 years, you might want to have the cannabinoids tested to see if it still has potency.
There is very little research currently on the speed at which tinctures degrade. As of now what we do know is that Tinctures can last for years retaining their specific characteristics. The taste, color, consistency, etc. may change but the integrity of the tincture will not.