About half the time, over the course of the World Championship era. It only happened once in the '70s, oddly.
I think and you might be able to tell me it's happened a lot more often since the 90's, that the driver who won the race has gone on to secure the title, I if I wasn't supposed to be getting ready for work I'd run the numbers.
It's only on something I remember them saying in commentary a few years ago.