Fully charging your dead iPhone using your computer's USB port takes anywhere from 90 minutes to all night, depending on several factors. To achieve the fastest charging rate, for example, turn off the phone and plug it in to your Windows computer's dedicated USB charging port, if it has one. The tech news website ExtremeTech reports that you can fully charge a dead iPhone battery in about 90 minutes under these circumstances. With a regular USB port on a PC, however, the iPhone usually takes closer to four hours to charge. The length of time increases when you leave the phone on or use it while it's charging.

Dedicated PC Charging Port

Most newer Windows PCs feature a dedicated charging port that provides a higher amperage than other USB ports, allowing your iPhone to charge at its full potential. This port is usually on the back of a desktop, and may be identified by a lightning bolt icon or by a different color in the interior of the port. The port isn't marked on some computers, so you may need to consult the computer manual or find the port by trial and error. If you have a Mac manufactured in 2007 or later, you can use any of the USB ports on the computer or display to achieve the fast charging rate.