How much longer does it take a car to stop on ice?

Assuming all else is equal (no specialized snow/ice tires, equal driver reaction time etc.) it will take a lot longer. Up to ten times longer. If the ice is on a downward-sloping hill, there is a chance that a car cannot stop at all, until it slides to the bottom of the hill or hits an object along the way down.

Winter tires, either studded or not, SIGNIFICANTLY help on ice, especially studded. Although Illegal in some areas/states, studded tires make driving on snow/ice as effortless and safe as on dry asphalt. If you're ever driving on slippery roads and come upon a situation where you need to brake hard, remember that if you slam on the brakes you will slowly start stopping, but you will not be able to change direction--you will keep going straight until you stop. I found that many times it's safer to not stop, and simply turn your car to the left or right (which is much easier/faster to do than coming to a complete stop), and simply go around the obstacle that would otherwise force you to stop. If you're at a busy intersection, this might not be good advice, but I'd still rather slide my car into a ditch alongside the road than rear-end someone and have my insurance rates raised.