Each smith machine has a different resistance. Depending on the pulley system used, each smith machine will provide more/less resistance. It's not black and white like weights where 50 pounds is 50 pounds.
I would say a good rough estimate is that the weight you can use on a smith machine would be equivalent to 70% the amount you can lift with free weights. For example, if you can lift 100 pounds on the smith machine, you could probably lift 70 pounds with free weights.
I really am not a big fan of the smith machine. It locks you into a range of motion and doesn't train the stabilizer muscles. Free weights are always superior. They will give you better results and they are safer because you have full range of motion.