It depends on what kind of debt you're talking about. If it's a credit card, the SOL is 6 years. If it's a more traditional written contract (like an auto loan, personal loan, etc.) it's 10 years.
More details here: http://www.bankrate.com/finance/credit-cards/state-statutes-of-limitations-for-old-debts-1.aspx
(Also, just to be clear, I'm not a lawyer and this is not legal advice - if you're worried about being sued, get thee to an attorney).
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