By default, the Nikon D3400’s Live View will turn itself off after 10 minutes. You can bump that up to a maximum of 30 minutes.
You can find this setting under:
Settings > Auto Off Timers > Custom > Live View > 30m
Where to Buy a Nikon D3400 DSLR
The D3400 is an excellent camera for getting started with DSLR photography. But now that it has been replaced with a newer model, it will become increasingly hard to find in stores.
You can also find them used at major used camera gear sellers such as KEH (which is where I often buy when I'm looking for used gear).