Cycling in Melbourne Australia
Its an interesting one ... ran into a couple of young cyclists (16 years old or so but guys who were training for the criteriums etc) at subway once and one of them mentioned that one of the pro's did it in about i think he mentioned 17 min ... which is pretty ridiculous when you think about it ... think going back to Walmer street from the roundabout is a little slower ... i took 10 min to get down to the roundabout and 13 to get back ... which implies some of these guys do the first leg in about 7 min possibly?
this should answer your question http://www.youtube.com/watch?v=I-ZN87-vjtc
Shane Miller did to laps of Yarra Bvd and did it in under 30min. The first lap took him all of 14.50. He's an A grade cyclist
You could also Strava or similar your ride, and quickly see your results against other riders.