The length of time or miles that tires will last once they have no tread depends and can vary.
I've had two tires on my truck before that were bald and had no tread left and I drove that truck another 2,000 miles before they got to the point where I really had to replace them before they blew out because the steel was showing through the tires.
Once you start noticing that steel wires are showing through the tire tread you need to replace the tires because they will blow out soon.
If possible replace the tires once the tread wears down because as tires wear down the steel gets closer to the surface and can't hold the tire together properly.
Always keep a spare tire with you and a jack and a 4 way tire lug tool to remove the tires when you need too.