For a photograph or other raster image, the useful measure is pixels per inch (PPI) at its final printed size. DPI describes the dots a printer produces. More printer dots cannot restore detail missing from a low-resolution image.
Standard expectations
Around 300 PPI at final size is a common target for images viewed close up. Large-format signage may need less because people view it from farther away. Confirm the target for your product rather than applying one number to every job.
Setting up your file
Set the document to its finished size and check the effective resolution of placed images. Enlarging an image lowers its PPI; changing a resolution setting alone does not create new detail. If you are unsure, send the original image and tell us the size you need.