Depending on when you go, of course.
However, assuming you are going at the normal work hour (Is there any more Normal work hour in this Internet age?), you will be looking at not much traffic from Mill Valley down to the Golden Gate Bridge. You will, of course, get a fast track pass for the Golden Gate Bridge so you can get a $1.00 discount from the toll and then use the fast track lanes.
You will then hit traffic on 19th street, but most of the lights are timed and there is no left turns there, so the traffic will be slow but steady.
After you pass 19th, the route to South S.F., whether by highway 1 or 280 are both clear.
So, I'd count 30 to 45 minutes to South S.F. during work hour.
But E. Blithedale and Sir Francis Drake can be tough also.