I live 12 miles from work currently. I work an 0800 to 1630 schedule. It takes me 40 minutes to go that 12 miles in the morning, and between 30-45 minutes to do it in the evening, though it has taken me up to 1.5 hours on days when multiple accidents were all over the highways.
You just can't take drivetimes for granted here, although of you work a 7-7 schedule your drive will be much more predictable. I have never worked at Wellstar, although if you do a search on this forum you will probably find some threads with info. Marietta is a pleasant place to live, it is considered a suburb of Atlanta and so yes, the traffic is still bad. I live in Roswell, where I have access to Northside, Saint Joe's, N. Fulton, CHOA.
If you want to live downtown, Atlanta Medical Center is another place to look.