At the traditional 96 dpi, you have to be 3 ft away to exceed the retinal density. Personally, I sit at half that distance. Something around 200 would be more ideal. Laptops you might sit even closer.

Mobile devices, unless you get really close to the screen, have matched the retinal density for a while. Most people hold the device at about 8 inches, so 450 dpi is the value to hit.

Edit These measurements assume 20:20 vision, which is the average. Many people exceed that. So you'd need slightly higher values if you're feeling pedantic.