Did you know that when you google "lifespan of potato" it gives you at least two options?
Raw and mashed.
Mashed they last 3-7 days (depending on the reliability of your fridge and the cast-ironness of your stomach).
Opinions vary on the longevity of your raw potato. According to wiki answers:
"Potatoes can be what we call an easy keeper when they are raw. If kept in a cool dry place they can last up to 2 months. As soon as a potato is dug up it starts to dry up".
Others suggest up to five months.
I guess it depends on what you want to do with your potato.
At about three months they look a bit like this:
(actually, I have no idea how old these babies were as I failed to log their arrival date in my potato storage bin. The Potato Office of Births, Deaths and Mashes is also missing in action).
But 5 months after that the potato starts to look a little the worse for wear.