Hours? No - thinking about a U of T student downtown. It's 12 km now from Kipling to St. George, with a 23-minute travel time. About 31 km/hr.
However, speeds would be higher further west, with a lot less stops. Let's assume 36 km/hr.
Extending to Square One is about 11 km. At 36 km/hr this would only take 18 minutes. Making the travel time from Square One to Yonge only 41 minutes for 23 km. Not only would it not take hours, it would only take less than one.
It's not an express train, but I'm sure that people would use it. Compare to the 14.2 km from St. George to Kennedy (currently 25 minutes - 34 km/hr!). The extension to Sheppard/McCowan would add about 7.5 km, another 13 minutes at 36 km/hr. Total travel time 38 minutes for about 22 km.
Extending the Bloor line deep into Mississauga (to Hurontario) is a similar trip and travel time to extending it to Sheppard McCowan.
If you dismiss one as taking hours, doesn't that mean we shouldn't do the other?