How long will it take for a measurable change in mass (corrosion) in an iron copper corrosion cell to happen? I am planning to do an experiment, in which I will test the affect of salt concentration on corrosion rate. Would there be a relationship? How long would it take for rust to form, that could be scrapped off and corrosion rate measured?
In order to get any measurements, you would probably need to leave the coupon in the solution for about a week or so. 

To see the difference due to the salt concentration differences, I would recommend going even longer, probably at least 2-3 weeks.