I used to have a 35mi round trip commute, 5 days a week. I rode it fixed, but that doesn't really change much. It was fun in the warmer months, but winter rides in ice and snow were pretty slow. I found that I didn't "take the long way" very often or have much motivation for weekend rides. I was also consuming noticeably more calories than my previous ~14mi commute.

I currently have a 16mi round trip commute. I mix up my routes and add some miles pretty regularly. I also have energy and motivation leftover for weekend rides.

Given that I ride year round and have to contend with a lot of ice or snow in the winter, I wouldn't want a commute over 20 miles round trip. If I lived some place without snow and ice, I think I'd be happy up to 30 miles.