Nearly 10 miles each way for me right now. I'm not a diehard yet so I only do it about 2-3 times a week now that the days are shorter. I get to work sweaty and have to do the whole cool-down and prep time. That is annoying in some ways. I also cannot be late for work so I'm wary of flats etc. I'm about to pick back up the frequency when my Lemond Poprad is put together.
I'm sure if the commute was about 3-4 miles shorter I'd ride every day. While not a "training ride" on the way in it would be a simple commute. Afternoons are for adding the miles.