Wildly inaccurate, but it really depends on the nature of the roads.

The route I rode today went through a lot of suburban residental neighborhoods with many stops signs and traffic signals (15-20 stops over 10 miles). Google maps estimated it at 53 minutes. I did it in 40 minutes, plus however long I was stopped, which I'd guess at 5 minutes. So that's within about 15%.

The route I rode yesterday went along mostly rural roads with little traffic and just a few stops. Google maps estimated it at 68 minutes. I did it in 42 minutes, plus however long I was stopped, which I'd say was less than 3 minutes. So that's right on the earlier quoted 2/3 error.

It also depends on wind and how much effort I put into the ride.