Depends on how you ride your commute, how fast you want to ride the century and the ride conditions (hills, headwind).

Generally I will consider myself ready for an event if I've trained up to 50-75% of what I need to do on the day, as long as I've had a rest day the day before the event. When I did my first century, I'd only done a little over 50mi as my longest training ride, but it wasn't an issue. I'm planning to try a 11000+ft century later this year and so far I've trained up to doing a ~45mi/6000ft morning ride without too much soreness the next day.