After 2000 images on my D600 I decided it was time to send it into Nikon to have the sensor professionally cleaned. The amount of dirt was just out of control and it was becoming too hard to "repair" every spot.
Below is an cropped example of the upper left corner of an enhanced image that shows off the amount of dirt/oil/dust that is stuck to the sensor. If you look at a larger view of the image, you can see at least 15 spots.
I took about 500 more "junk" shots to get the count to 2500, packed up the camera and shipped it off for repair. I hope Nikon is fast.