Twice so far. I'm aiming for once a week, which may seem pretty lame to those who are already doing it. Maybe I can bump that up to twice a week.
I only live 3 miles away from work, but the roads and traffic is horrible, and I was terrified to risk my life. I'm happy to report that it hasn't been as bad as I feared. I found routes that are about 4.5 miles long, and put me on roads with heavier traffic only 30% of the time. The worst issue I have is a headache and sore lungs from the fumes (I have chemical sensitivies). I plan to try wearing a mask next time on the fumier roads.
I only got on the sidewalk a few brief times, where there was simply no safe way to stay on the road (no bike lanes in town, no shoulder whatsoever, tons of potholes...). One of the times I took to the sidewalk I just walked the block until I could get back to biking on the road.
Everything everyone has said is correct - you just have to give it a try. The perceived excuses are not insurmountable. I think I was finally pushed into action after watching James Cameron's 'Years of Living Dangerously'.
I feel like this could be the beginning of a great habit!