Had the ignition switch replaced in my 2002 Boxster. It was not clear it was the problem but I hate a car that is unreliable and not starting at random times is my definition of not reliable. So I was willing to throw a new switch at the behavior.
After having suffered from trying to fit myself under the dash to replace several of the various switches related to the clutch interlock, cruise control and brake lights, I opted to let a Porsche tech do this ignition switch job and of course the entire switch assembly -- the electric blob on the end along with aluminum assembly was replaced. I can't recall the mileage now but it was sometime in mid 2009 or at 7 years of age so I estimate at around 150K miles. At 15 years old and with 317K miles on the odo the replacement switch was just fine.
The techs told me that just replacing the electrical portion resulted in too many bring backs so Porsche no longer offered the electric portion separately. I did buy one from the next door Audi dealer but never installed it. I gave it away after I got rid of my Boxster.
I mention the above in case you just replaced the electrical portion before. The delayed shut off reads more like a mechanical problem than an electrical problem.
Pedro brings up a good point about a heavy key chain. I never used a key chain I always had my Boxster (and Turbo) keys separate to avoid putting any undo strain/wear/tear on the ignition switch. A habit I developed when I rode motorcycles and didn't want a heavy key chain flopping about.