8.5 miles to 17.4 miles depending on weather, season and mood, but most of the time 10.6 miles. Going to work is definitely faster/easier as my best time for the 10.6 miles is 33 minutes (I'd say the average is 36 minutes), best time on the way back 38 minutes (average about 42 minutes).
I commute on a MTB with Schwalbe Hurricane tyres so that I can go on or off-road depending on the factors metioned above, but most recently I've stuck to the roads so from next week I think I'm swapping to the single speed.