Does it even matter that it takes three months? Most groupons are valid for more than three months. Even though I assume most are used within the first month, its still not all of them.
He was probably talking about lifetime earnings and not yearly income. If you factor in the 11-19 years of additional training and lost income that doctors go through and make a plumber work the same amount of hours as a doctor, then I bet their lifetime earnings will be close.