By default, the iPhone will lock the screen and keyboard after one minute of inactivity. If this is too short (or long) for you, you can adjust the amount of time the iPhone waits before it locks itself. To adjust the auto lock, tap the Settings icon on the Home screen and then tap the General button in the main settings screen. Tap the Auto-Lock option and you'll be given various options that you can select for how long it takes for the auto lock to engage... one of which is never.