Sadly a patent examiner (in the UK at least) gets less than a day to do a prior-art search on the most complex applications in the most complex subject matter areas; half a day is probably the standard.
There are half-a-million patent applications per annum in the US alone (10 million since 1963). I'd estimate a global corpus of billions of patent applications. Prior art search extends though [in theory] to all publications.
You can see why prior art submissions are allowed from third parties. If you want to ensure only valid patents, or perhaps as few patents as possible, are granted then making such submissions is in your own interests.
Edit: Actually the gross figures are misleading, but remember these are complex 20+ page [in normal print size] documents on the whole. G06F11 - an IPC classification concerning error correction, detection and handling has ~9000 docs in the espacenet database for each of 2010/2011/2012.
