My commute was 27 miles one way. I was typically leaving a car at work, cycling home and back in the AM. This was partly on designated bike lanes in Brooklyn, then bike paths, then many local streets. As it was rush hour, I made a conscious effort to not hammer or otherwise make it a training ride. You really, really need to pay attention and racing to make a traffic light is a surefire way to get hit. If you need more mileage, figure out some variations on the way home when you've got the time and extend to whatever you feel comfortable with.