My experience of 10-15 is the extra time it takes over the short term carpark option, total. Extra time on the bus in transit is offset by the time it used to take to drive up seven levels of carpark and walk down again.
Yes, I know, it's still an extra 20-30 minutes extra a day even with my numbers, which is annoying. I just hate the mindless exaggeration that seems to pervade some of these forums. This 28 minutes each way thing for example. Why not just stick to the verifiable facts and let them speak for themselves?
Wasting my breath no doubt. Hat, coat, door.