When does mirena (levonorgestrel) stop working?

5 years. The Mirena (levonorgestrel) is approved to be effective for 5 years. While protection may last longer than that there is no way to know for sure. Don't take a chance. Having an iud still in place if you get pregnant creates significant risks.
5 yrs, possibly 7 yr. The Mirena (levonorgestrel) IUD is one of the BEST forms of birth control available, it lasts 5 years, is ~99.5% effective, and thins the uterine lining so that you get lighter periods. 20% of women will stop having periods after 1 year and that's perfectly healthy. Effects in the uterus and so it won't affect or be affected by other medicadtion; very safe method; biggest risk after insertion is expulsion (1-2%).
5 years. The Mirena (levonorgestrel) iud is fda approved to remain inside the uterus for 5 years. It can be removed and you may have a new one placed at the same office visit. The paragard iud can stay in for 10 years. I hope this helps. Wishing you the best in health.