It depends on the printer. Check with the model for the optimal PPI setting. 300 PPI is standard but many inkjet printers will print fine at just 200 PPI. It depends on the content, the details on the image, and the viewing distance. For example, a tight crop beauty shot will print very well at 180-200 PPI even when enlarged to 6 feet by 4 feet because it will be viewed from afar. A mosaic image using many small images to create a large image will required 300 PPI because each individual image must be rendered clearly and those images will be viewed at a close distance.