19 miles one way for me. At the moment I think 19-23 is probably the upper range of what I would commute.

Doing it twice a week now, last week I tried to do it 3 times, but unfortunately the 38 miles wears me out to a point where I'm still not able to do it on subsequent days.

Also one of the issues with anything longer than 10 miles, in my opinion, is that it becomes really demoralizing when conditions are miserable. Both commutes home last week were directly into 20mph headwinds. After a long day at work I end up cursing the whole way home thinking "why am I doing this?!".