By default, the Nikon D3300’s Live View will turn itself off after 10 minutes. You can bump that up to a maximum of 30 minutes. This come in handy when you need to keep the Live View active for longer stretches, such as when you’re connecting your D3300 to a computer to use as a webcam.
You can find this setting under:
Settings > Auto Off Timers > Custom > Live View > 30m