With the right infrastructure, 12 miles is about a 45-50 minute ride, for what it's worth. When I commuted from Santa Monica to El Segundo it was 13 miles and the bike averaged ~5 minutes slower than car (I was in the best shape of my life and loved starting the day that way, to boot.)
At lot of that depends on where you live. Southern California has a pretty ideal climate. I'm in Austin and I can manage my 7 mile commute in the summer until the temperature gets above about 102. I'm fortunate that my workplace has a shower.
A 12 mile winter commute in some of the northern states could be pretty dangerous.
