Shania Twain has one album that went to 19 platinum (million) and another that went to 12. The one that hit 12 took 5 years to get there from gold (half a million). The one that went to 19 ALSO took 5 years to make the climb from gold to current cap (
http://www.riaa.com/Gold-Intro-2.cfm you'll have to plug in for your own searches the results page doesn't make unique or reproducable URLs ). Definitely not averaging 20 mil, and taking a lot longer than Kelly has even been around.
Faith Hill has one that went to 7 plat, one to 5 and two to 4. Got some numbers problems with your estimations.