There are just too many variables for a simple answer. The type of bike you're riding, the wind speed and direction for the day, the number of hills or elevation you'll be riding, what you ate for breakfast that day, if your tires are inflated properly, the number of traffic lights and stop signs you have to deal with, etc. all have an impact on how fast one rides. Perhaps the question should be, "How fast do you want to ride?" Do you have a goal in mind? If not, why even think about it?