I would say: assume it will take you 30 minutes, unless the road is perfect. Assume more time if you climb some serious hills. That assumes an average speed of 8 miles per hour or 15 km/h. It sounds quite low, but takes into account red lights, time you slow down for traffic queues, etc.
ONce you know the road, the time will decrease not only because your legs will have more training, but also because you'll know when to race to catch one of two extra green signals in a row, or when to slow down and relax because you would have to wait for a red light otherwise.