Like others mentioned, time is the limiting factor for most people. My commute increased from 22 to 30 miles round-trip in October. The distance isn't an issue, but I had to start leaving about 30 minutes earlier and I get home that much later in the afternoons. Getting up at 5 a.m. is tough. My "official" time is about 1 hour each way since my computer stops counting when my bike stops, but actual riding time ranges from 60 to 75 minutes depending on wind, the number of red lights I catch and how tired my legs feel.