Ummm not long can't remember tbh but I ride Windsor to Waterloo once or twice a year on the path. Maybe 1/2 hour to 3/4hour but its easy to get off and go in the road if you need to. Weekends are busy with foot traffic as well.
That'll really add some time on to a commute I would think. It would also be a good idea to ride it a couple of times to check it for yourself, but looking on a map you can see how much it stretches the journey.