By default, the Nikon D3500’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 D3500 DSLRThe D3500 is an excellent entry-point camera for getting started with DSLR photography. You can find them at Amazon and B&H Photo. 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).
Nikon D3500 Tips
I've put together a number of resources related to the Nikon D3500.