While the link Kona gave was good for info on breeding the peppermint shrimp, I don't think it talks about actual lifespan of the organisms. Life-span is mentioned once in the last article, and I believe it is in regards to the lifespan of the cohorts (meaning companion groups)... I believe that this is refering to how long the pairs last? Peppermint shrimp longer than 12-18 months. I believe their true lifespan is several years, I would guess around 5 on average, but don't have a better statistic with a reference.