Nearly one in five (18 percent) prospective first-time homebuyers said extending the $8,000 tax credit would be the primary influence on their decision to buy a home before the end of 2010, according to a Zillow(R) survey. That would equate to 334,000 buyers from Dec. 1, 2009 to Nov. 30, 2010 - a likely time period for an extension, according to additional analysis.