The amount of time you can go without paying property tax before the county sells the property at the tax sale is at least 6.5 years.

I researched this on the county tax collector site (http://www.sdtreastax.com) after I found a property that has gone 5 years without paying the property tax.

Five years after the property is in default it becomes subject to the county’s power to sell. It isn’t in default until the end of the first year with no payments. So the first year of not paying is free. Then, after five more years of no payments, the county can sell at the tax sale (and must within 3 years). The tax sale is in February and July is the default month. 6.5 years from last tax payment made to tax sale date can be safely assumed.

The June 8, 2009 Notice of Impending Power to Sell Tax Defaulted Properties does not list the property I am watching (last current in June 2004, no payments made 2005-2009). The most recent properties to be served notice: PROPERTY TAX DEFAULTED ON JUNE 30, 2004 FOR THE TAXES ASSESSED
AND OTHER CHARGES FOR THE 2003-2004 FISCAL YEAR.

This means the taxes for these properties were last current on June 30, 2003. They will make the February 2010 tax sale.