How old is radar imagery when it is received via ADS-B?

ADS-B offers regional and CONUS NEXRAD radar. Which one you get will depend on the tower type from which you are receiving.

CONUS radar images, which are transmitted over Hi- and Medium-Altitude tier ADS-B towers for the lower 48 states, are updated every 5 minutes, but the images are only transmitted once every 15 minutes. This radar imagery has a large pixel size. 

Regional radar, which is is provided by each ADS-B tower, is updated every 5 minutes. ADS-B transmits the images every 2.5 minutes, but the data only changes on a 5-minute schedule. In clear air conditions, where there are no radar returns, the update rate is extended to once every 10 minutes. 

Depending on which towers are being received, a mosaic will be assembled with the regional and CONUS products.