My longest commute was 17.5 miles each way, into and out of a valley, with lots of hills along the way. I only did that for a few months, but with the hills, it took about 1-1/2 hours, each way, on a fixed gear bike. My current commute is 13.1 miles each way. I love my commute, it's a mixture of diffferent topography that keeps me engaged. It takes me around 50-65 minutes, depending on the traffic and headwind conditions.

I wouldn't do more than 50 miles, round trip.