I did some web searching and this seemedGoogle purchases the images from many different sources. Most of the images are purchased second hand - basically the images are not taken for the sole purpose of supplying them to Google. Most of the images are taken for completely different reasons and released to the public or sold to Google for extra profit.
Suggestion - your local government agency (county or city) should have a Geographic Information Sytems (GIS) department or division. They could be a great source of aerial photography.
I work for my local GIS division and we have images at 6" resolution taken in spring of 2008, 2007, 2006, 2003, 2001, and 1997.
Check with your local government - they will be the best source for updated imagery to be a "best" answer to the question on updates and resolution.
I don't really know how often the images are updated. More of an issue seems to be the resolution of the area. Some areas, for example where I live, are extremely high resolution and you can make out very close detail. In other areas, those more remote, the resolution is so poor as to spoil the view of whatever I am trying to see. OTOH, I've found resolution on Zillow to be even better than Goggle Earth in some areas. Just have to look and see how often they update and how high the resoution is in the area you are keeping tabs on.