Some are pretty recent, some are years old, my parents farm hasn't been photographed since 2003. Other city areas are more often. Google gets the images from multiple sources such as navteq and the usgs as well as other state and local gov and even private surveying companies. The USGS has imaged the whole country via satalite then other entities contract airplanes to fly over with cameras and those are overlayed to make more up to date and accurate images.
The closest to real time is when there is a atural disaster and someone either gets the satalite to image it or flies an airplane over it, but those are usually for specific instances such as the coveraga of hurricane katrina but never make it down to google unless google were to then request the data dn process it and add it as an additional layer in earth. Your still talking about days worth of work.