It depends on where you are. Google doesn't actually take the pictures. Most of the pictures are 'digital orthophotos' taken by cameras mounted on satellites. They come from various sources that can be traced to world wide government agencies because private companies can't yet afford to launch satellites. In some places, however they will use 'high resolution aerial photos' when available. These are taken from airplanes, hot air balloons, etc. in smaller areas for urban planning, etc. These photos will have incredible detail.
When are they updated? The answer is, when they become available. For example, I live less than 60 miles from New York city and the Google maps of my house show a swingset I tore down 4 years ago. They are not usually real time photos, but you never know when new ones will become available, so it is pot luck for the end user. All the other aerial photo sites (Mapmart, Digital Globe, Flash Earth, etc.) get their photos from the same sources. What makes one better than the other is how well they keep track of updates. The whole thing is way more complicated than I can describe, this is the gist of it. I hope it's helpful
I am a Land Surveyor. Aerial photogrametry and Geographic Information Systems (GIS) are a big part of my business.
Ronnie