http://www.theguardian.com/politics/2014/apr/23/boris-johnson-intentions-next-election

Great quote from Labour's Shadow Minister for London (and likely Mayoral candidate in 2016), Sadiq Khan:

"Johnson has stopped hiding the fact that he is a part-time mayor. The greatest city in the world needs a full-time mayor committed to London. Johnson has simply lost interest."

Here's Sadiq with another part-time Mayor: https://www.youtube.com/watch?v=Zx7kyrSk6WE