3 - 4 days
||Martínez DE, Bridge D. Hydra, the everlasting embryo, confronts aging. Int J Dev Biol. 2012 56(6-8):479-87. p.480 left column 2nd paragraphPubMed ID22689361
||David CN, Campbell RD (1972). Cell cycle kinetics and development of Hydra attenuata. I. Epithelial cells. J Cell Sci 11: 557-568.PubMed ID5076361
||p.480 left column 2nd paragraph:"Not only do the interstitial stem cells give rise to both gametes and somatic cells, but, unlike the somatic cells of most animals, Hydra epithelial stem cells have the ability both to divide endlessly and to differentiate into several cell types. A well-fed Hydra has an epithelial cell turnover time of 3 to 4 days (primary source), so that an individual Hydra could have all its epithelial cells replaced within a week (Martínez 2002). This unique ability to discard and replace older cells, and to rejuvenate its soma has led scientists to propose the lack of senescence in Hydra—an idea championed by Brien (1953) who reported keeping individual Hydra alive for five years without observing any signs of aging or reduction in budding rates."