My commute is about 13 miles, but I often extend it to 14 or 15 on the way to work. I'd probably do up to 18 if I had to. I think the afternoons - especially hot summer afternoons are when it is toughest for me.

Like others mentioned, you have to consider the time involved. It's possible physically to ride that much, but you'd be getting ready to ride, riding, and cleaning up after for close to two hours each way. You would also have to be pretty diligent about bike maintenance as putting that many miles on will wear things down faster.

Good luck!