I'm commuting ~10 miles RT for work, four days a week and probably an extra 5 miles for school and other stuff three of those days. If it I wasn't crossing some of the busiest roads in my city I'd probably get from my apartment to work in about 15 minutes, but I spend at least 5 minutes stopped at various lights around town that intersect heavily trafficked roads. In the spring, fall and winter the rides aren't too bad, although riding home at 12am in 20 degree temperatures isn't something most Texans are prepared for. The summers are a totally different monster, once the temperature is hitting 100-105 daily in July my 20-25 minute ride turns into a 25-35 minute ride (I think my hottest ride this summer was 114 a few times) and the heat sucks a ton of energy away. Additionally most of the year I just have to change clothes when I get to work, but from June until mid September extra time has to be allotted so I can stop sweating and get all cleaned up. It takes anywhere from 15 to 30 minutes to get from my apartment to my job via car so I'm not really saving or losing any time by riding.
I think it's all situational, right now I'd have no problem commuting 20-25 miles round trip for work but through June, July and August 20 miles daily might be pushing it in the heat.