What determines how long you hold down an alphanumeric key before the keyboard will repeat the character?

The duration for which you hold down an alphanumeric key before the keyboard repeats the character is determined by the keyboard's "repeat rate" settings. This is typically adjustable in the operating system's keyboard settings, where you can set the delay before the repetition starts and the speed at which the character repeats. Factors such as the keyboard hardware and driver may also influence this behavior.