How do cave ins work?

I heard you need to build a mine support where the cave in happens but how much time do I have to do that?
What if I dont build a mine support? will rocks fall on my head or the mine be unusable or..?
Does a mining helm protect me from the cave in?
also how safe am I when I mine within the range of only one mine support?
What if I dont build a mine support? will rocks fall on my head or the mine be unusable or..?
Does a mining helm protect me from the cave in?
also how safe am I when I mine within the range of only one mine support?