How long does it take to propagate a newly created block to the entire Bitcoin network and why? Once a new block has been created, it is propagated to the entire network. I’d like to know how long this would take “on average”.

The propagation delay can be from several causes, and I’d additionally like to understand which one is the most significant:


As I understand, blocks are not propagated before being verified and accepted, so this should contribute to propagation time.
Network latency adds delay regardless of the block size.
Bandwidth limitations: large blocks will take longer to transmit, and nodes with many outgoing links will take longer to transmit the block to all their neighbors.
The network’s diameter.


If there are other causes of delay I’d love to hear about them.

Also, is anyone monitoring the network’s propagation delay, or any of the other network measurements I mentioned, and if so, where can I find statistics?
According the paper 'Information propagation in the bitcoin network.' by Decker and Wattenhofer. The mean time for a node to see a block is 12.6 seconds, after 40 seconds 95% of the nodes have seen that block. This paper goes into some of the causes of propagation speed.

Block size is a dominating factor:


  "For blocks, whose size is larger than 20kB, each kilobyte costs an
  additional 80ms delay until a majority knows about the block." -'Information propagation in the bitcoin network.'


Network Diameter does not appear to be that important as there are some very long shortest paths in bitcoin, but those paths only represent a tiny faction of the overlay network.
The bitcoin network appears to be extremely well connected for most of the nodes and with a long narrow tail very very poor connections. A block may take a long time, possibly minutes to reach the last 0.0001 percent.

Bitcoin Stats have a nice website that measures block propagation speed, it is based on the research of Decker and Wattenhofer. 
It doesn't entirely agree with the Decker and Wattenhofer paper but it is important to understand that the bitcoin network is evolving at a breakneck pace. The bitcoin overlay network like the weather or the stock market, has the property that what was true yesterday might not be true tomorrow. 

Bitcoin Stats says that for today, the mean was 5.1 seconds and 90th percentile was 22.4 seconds.