NEW YORK (CNNMoney.com) -- The tell-tale sign of a stagnating real estate market? When homes for sale start lingering - and that's exactly what real estate brokers and other industry watchers say they're seeing now. The National Association of Realtors does not maintain national time-on-market figures. But inventory - the number of homes for sale - spiked 37 percent for the 12 months through April 30, the most recent data available. Homes are staying on the shelf longer. Languishing in hot markets There are no official regional statistics for the time homes spend on the market. Here are estimates for...