How does Google Maps estimate cycling times
This site seems to have some good answers to your questions. It says


  Google assumes a baseline moving speed of around 16km/hr (10miles/hr) regardless of trip distance.


but if you read more you can see there are adjustments to that baseline. For some routes where I've actually compared, I divide the Google cycling time by 1.5 to get an estimate of how long it would take me, but I used to race pretty seriously.