I'm at about 97,000 miles right now. I don't rmr correctly but when I bought the car I feel like the car fax stated that the car got new spark plugs at 60k but not sure. I've been meaning to stop by the autostore and check the OBD reader but just haven't had the chance.
I did however have a talk with the mechanic close to my house and he says with spark plugs it's a gradual decrease in mpg instead of overnight but who knows? Lol