You can drive on a bald tire until the steel starts wearing through or until the tire blows out.
If you have bald tires though then you really should have new tires put on the vehicle before it causes a car wreck.
Bald tires do not have as good traction as new tires with tread on them do so without the tread and traction it takes longer for your car to stop when you apply the brakes.
Also when you're driving on wet roads the bald tires can cause you to lose traction as well and then you may hydroplane or go off the road or slam into someone else's vehicle.
If your vehicle is a front wheel drive vehicle and you have bald tires on the front you should put the bald tires on the back and have better tread on the front.
From my experience when a tire goes bald I could usually drive another 500 to 1000 miles until the steel shows through and then it goes completely bad.
I used to have to run tires until they got really bad because I was poor then but now that I have the money I replace the tires on my vehicle when they start getting bald for safety reasons.