How long would it take to terraform Mars?

It's likely to take hundreds of years, as you've got to change the composition of the atmosphere of a whole planet. You need got to undo the reasons that Mars doesn't have an atmosphere already, which is that it doesn't have a magnetic field. The solar wind has gone past, but there's no magnetic field to deflect it, so much of the atmosphere has been lost. We're still looking at probably 30-50 years before we send a man to Mars, which we will almost certainly need do do before we even think about starting to terraform Mars.