Clostridium tetani (C. tetani) is a bacterium commonly found in soil and is excreted in the faeces of many animals (both mammals and birds) and serves, by means of the exotoxin, tetanospasmin, causes the life-threatening condition tetanus.
Soon after asking this question I found a reliable source, the National Institutes of Health. It can survive for over 40 years.
Clostridia form endospores and can survive for years as a dormant spore. You can check this article about viability of clostridial endospores but it doesn't really talk about how long it can survive.