Everything said above. My ride home, using the very same route, takes about 2/3 the time of my ride in because I'm going down the durned hill instead of up. If I do the ride on the weekend, it takes less time than during the week. Riding in full light in the summer is faster than darkness and rain in the winter. In general, traffic and environmental changes can add or subtract as much as 25% to/from my ride times.

Given a relatively flat route with minimal traffic and stopping, and assuming you and your bike are in reasonable shape, you should be able to do a 15 MPH average, so 12 miles would take about 45 min.