Google Now gives absurd times for arriving on time
Have you enabled Location Services? Google Now determines the time you may take to get to your appointments from your synced calendars and your current location. I am sure you have synced calendars because Google Now reports an appointment. Location Services maybe an issue. If you are using the stock ROM for your device, enable Location Services and sign into Latitude to set your current location. Setting your location manually may also solve your problem. If you are on a custom ROM, you can set the device up to use only the Internal GPS to report your current location to Google Now. Hope this helps. If you give this a shot, report back with results for others to help you or be helped.

P.S.: This may be useful.