With the Volt? 6 months or so. With the Cadillac? At this rate... about 3 weeks give or take. Funny how that works!
Fun fact: I reset the MPG computer after I fixed the intake leaks and ran some fresh fuel through the Caddy. As of today my average reported MPG: 12.6. I've got ~250 miles on it since the repairs, so plenty of time to let the computer sort things out. I've confirmed via scan data that I'm running +/- 3% on my fuel trims so she's running right where she should. Of course, those are in town numbers. I've hardly had it on the highway since I bought it.
On the flip side, I let my buddy borrow my Volt to go to Green Bay to watch the Vikings game this weekend. He averaged 40 MPG over the trip, easily beating the 37 MPG rating. It cost him $80 to go from Minneapolis to Green Bay and back. Compared to his Suburban, it saved him almost $150 in a single weekend.