How often does Google Earth take pictures?

Really depends on the source of the satellite imagery.

google for examples purchases or leases its satellite imagery from a number of imagery providers such as GeoEye and DigitalGlobe. google updates it imagery on google Earth and google Maps once or twice a month but not the entire globe at once.

Bing Maps (previously Live Search Maps, Windows Live Maps, Windows Live Local, and Microsoft Virtual Earth) is Microsoft's web mapping service. New updates are released on roughly a monthly basis.