Optical CPU - Optical or Photonic Computing

My estimate is 20 years.

http://www.zurich.ibm.com/pdf/nano [...] cts_en.pdf

In practice, optical chip-to-chip connections will first be applied to high-end computing. Scientists
are optimistic that this technology could be ready for use in commercial systems within a few years.

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The Photonics group in Rüschlikon is working in close cooperation with related IBM
teams on several other innovative projects aimed at integrating optics on chips. One
project involves using silicon nanophotonics to integrate optical functionality into
processors on the nanometer scale. First devices demonstrate the potential to
achieve data transmission rates in the range of terabits per second.
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First prototypes of this technology are already functional in a laboratory setting, and
could go into standard testing within the next ten years. Based on the current trend
toward ever more powerful high-end computers, it is expected that supercomputers
comprising nearly 100 million processor cores and featuring exaflop/s performance
rates, i.e. 1018 operations per second, will be developed within that timeframe. Without
improved energy efficiency on all levels, however, such supercomputers would
require their own dedicated power plant. Optical data transmission will therefore be a
key technology in the development of future IT systems.