Again on average.
People tend to ride an average of 50 km/h without stops.
Besides this i think most riders do know there average speed.
When i ride with a (little) group i ride 50 km/h without stops and 30 km/h with stops on average.
When you know your average speed. I becomes very easy to calculate.
Also, the routeplanner give you also a pretty good indiction of the riding time.
A few times i did a tour where the start, lunch and diner times where set in a certain windows. It is upto the rider to be there in time. From what I saw, hear and read. Nearly everyone knows there limits.
These tours are tours with more than 200 people. With all the possible motorcycle and ridingstyles you can think of.