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

When connected to an ADS-B receiver, pilots can receive Regional and CONUS NEXRAD radar. Radar type depends on proximity to returns and the towers you're receiving from.

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. CONUS radar is low resolution and depicted with large pixels. 

Regional radar, which is is provided by each ADS-B tower, is updated every 2.5 minutes. In clear air conditions, where there are no radar returns, the update rate is extended to once every 10 minutes.