If a six-month supply of homes for sale represents a good balance between demand from buyers and sellers, homes in balanced markets will typically be listed for about six weeks before they sell, the National Association of Realtors said today in announcing a new metric, median days on market.

As it turns out, NAR statistics show median days on market is down 29.6 percent from a year ago, from 98 days in July 2011 to 69 days in July 2012.

While days on market can be pulled from statistics maintained by local multiple listing services, the task is complicated because sellers sometimes take their homes off the market and relist them.

NAR is tapping monthly member surveys the trade group uses to generate its “Realtors Confidence Index” to calculate the new median days-on-market figures, which will be included in future existing-home sales news releases.

Median time on market includes all listings, which can be misleading if an influx of fresh listings skews the calculation downward, NAR noted.

From 1987 through 2011, time on market was typically 6.9 weeks, NAR said. During that time, NAR’s data on existing-home sales showed the supply of inventory averaging 7.0 months, indicating slightly less demand from sellers than in a balanced market.