I haven't tested it yet, but my usual 13.3 mile exercise ride takes me about 1.5 hours. And Google maps gave me four routes of between 17.1 miles and 15.2 miles, the 16.2 mile route is quickest with a 1 hr 41 minute time, while the 15.2 mile route was listed at 1 hr and 49 minutes. I assume that this because the shorter route is hillier.

From what I've tested, Google Maps times are really close to my actual road times. I am marginally faster, but for planning sake, at least I can assume that these times aren't too optimistic. But I also suspect that with practice, my ride times should drop a little. And it looks like I'll have to try all 4 basic variations to know for sure.