How long would I have to be using a lash serum with a prostaglandin analogue before noticing darkening of my green eyes?

The concern about eye color change is associated with lash serums containing prostaglandin analogues, which can potentially cause darkening of the iris. However, this side effect is generally associated with long-term use rather than short-term exposure.

Duration for Color Change: Typically, the darkening of the iris, if it occurs, is linked to prolonged use of products containing prostaglandin analogues. It is unlikely that using such a product for just three days would result in noticeable changes in eye color.

Previous Use: Since you used a whole tube of Neulash in the past without any change in your eye color, it further suggests that your eyes may not be particularly susceptible to this side effect.

Current Risk: Given the short duration of your recent use of RapidLash, the risk of experiencing a change in eye color is minimal. It is reassuring that you stopped using the product promptly upon learning about the risks.

Monitoring: If you are still concerned, continue to monitor your eye color. However, any significant change would likely not be noticeable after such limited use.

Consultation: If you have any ongoing concerns, consider consulting with an ophthalmologist for peace of mind and a professional evaluation.

Overall, while the potential for eye color change exists with prolonged use of prostaglandin analogues, your brief usage is unlikely to have caused any permanent effects.