3 days per week on average, and it's a 50/50 commute for me: Drive to the park 'n' ride, ride to work.
My round trip route is 32 miles and I start at awful o'clock in the morning. To get in on time, shower, have some breakfast, etc. I'd have to get up around 3:30 in the morning. Screw that. I still get 20mi r/t by doing the 50/50 commute, and cut my overall driving by 50%, plus I can sleep for an extra hour.
Mostly it's the weather keeping me from 5 days right now. I've had enough of this crapola rain/sleet/hail mix we've been getting all winter/spring.