The serum should not stop working and there is no evidence clinically that would suggest individuals become immune to the formula. However, because our biology is unique and our systems are constantly going through phases of growth and repair, stress and life, we have noticed that there are seasons when visible changes like less robust growth occurs. Our Founder and CEO, Leah Garcia has been a go-to for understanding the ebb and flow of results. For example, she too experienced a time-period after using the serum for over two years when her lashes were just not responding as magnificently. She stopped using the formula for 30 or so days and then, re-introduced it. Her lashes not only kicked into another natural growth-spurt, they came in luxuriously feathery and symmetrical. We don't have a definitive timeline on how long to rest, and there is nothing scientific about this troubleshooting tip. It's just a bit of real world creativity.