Good guidelines above, although they're a bit too finicky for me. As a general guide, the food item needs to be about one and a half times as wide as the snake's body (not the head or neck). It's about the right time to try the next size up, when the food bump has gone within about 24-48 hours.
As for how long your Corn will be eating pinkies - that's difficult to predict. They can grow at very different rates. I'd reckon on a few months, but then I've tended to grow mine on very slowly.
If you find that you've ordered too many pinkies, you can always feed them two or three at a time until they're used up, when the snake's big enough for a size increase.