I was really looking for anecdotal evidence, rather than hard facts - just something quick and dirty to look at so I could get a rough sense for comparison. I used this service (http://www.xefer.com/twitter/) to quickly look up a few accounts.
Above is the @680News twitter account. As you can see, their distribution is clustered around the 5-am to 11am spots on Tuesday through Friday. Then again 5pm to 8pm on Monday through Wednesday. No idea where they are on Monday's or on Friday afternoons.
CBC News seems to start at 5am and be done by 2pm... except on Mondays when they keep going until 6PM. It would be interesting to know what happens on Tuesday's at the CBC - is that "meeting day"?
Here we see with CityNews that Monday's are 5am to 5pm in earnest, then like the CBC, they relax on Tuesdays. Also like the CBC, there's a definitely pattern Wednesday through Friday 5am to 1pm (CBC stops at 2pm).
This chart for CTV Toronto surprised me most. They run 7am through 8am, then take a break at 9am. 10am through 1pm is heavy, but they keep going through 4pm and then break again at 5pm. At 6pm, the third push of the day happens. What really stand's out is that pattern at CTV Toronto, whilst fainter, is still discernible on weekends.