Unless you are familiar with the route google.maps is suggesting, I suggest doing an actual ride. Google.maps is famous for inventing roads. The last time that I used it on my phone for navigation, it tried to send me down a private road that was closed to public traffic, and I wound up having to circumnavigate through the bad part of da hood. Not fun.

Time wise? It's actually fairly accurate for some routes I know are correctly mapped. The number of major intersections that need to be crossed and traffic lights and traffic on narrow roads tends to limit my speed.