For comparison Crossrail project really got going in 2001, with legislation in 2005 and construction started in 2009 and some services will begin in 2015.
Bakerloo line would be a little more complicated to start running services as they would need to fit with existing services, but this shouldn't cause more than a couple of years delay.

If there was the political will to push forward with the Bakerloo line extension it could be running in 15 years (2030). But with other projects such as Crossrail 2 and HS2 sucking up all the transport planning and investment, it seems unlikely that anything would move forward significantly in the next 5 years.

The most significant words in Darren Johnson's quote is "...should be extended beneath London’s packed streets". This is exactly what I have been arguing for the last few years, rather than extending the Bakerloo line on existing rail tracks.

From this point forward I shall be referring to the overcrowded 176 as the Bakerloo Extension Bus Replacement Service!