If you are making images for commercial publication or other critical applications, or if you want exhibition-quality prints from commercial printers, divide the pixel counts by 300.
Let’s take an image from the Nikon D90 as an example. It makes a high quality jpeg image at 4288 x 2848 pixels. Using the formula above, that would generate a native print size of 17.8″ x 11.8″ at 240 DPI. Now remember, this assumes you haven’t "pumped" up the image in Photoshop.
The next step in the formula is to take this print size and multiply it by 25%. That is the largest amount of image interpolation the picture can stand and still maintain reasonable photo quality - in my opinion.