By default, the iPhone automatically goes into sleep mode after one minute. It is, however, possible to increase the amount of time before an iPhone will go into lock mode. Many iPhone users choose to increase the amount of time before this auto-lock feature activates to prevent iPhone applications from inadvertently closing when the iPhone goes into sleep mode. You can easily increase the lock time of your iPhone through the Settings menu.
Tap the "Settings" icon.
Select the "General" option and then tap "Auto-Lock."
Select the amount of time you want your iPhone to wait before automatically entering sleep mode.
The iPhone's battery discharges slower when the iPhone is in standby mode. Therefore, increasing the amount of time before the auto-lock feature kicks in, will potentially reduce your iPhone's battery life.