Depending on my route, it's 18 miles (29 km) or 12 miles (19.3 km) each way.
The cyclometer usually shows my avg between 12.5 mph (20.1 kph) and 13.5 mph (21.7 kph). However, coming into work is a bit more uphill than going home, so it regularly drops to as low as 10 mph (16 kph) in the mornings.
I think my record average in getting home was 15 mph (24.1 kph) and I've only pushed that hard once .


(edit) - My commute regularly takes somewhere around 1 hour and 45 minutes up to 2 hours.