I'd say Atlanta is worthy of a vacation of say a few days. I'm kind of hoping to drag my family there to the aquarium (I was there on biz so the kids missed out on the aquarium
). But I'd probably combine it with a trip to visit some friends up in the Carolina's or something like that.
There's also the Coca-Cola museum. Time was limited for me so I couldn't check it out but it's right next door to the aquarium. Think they also have a zoo.
So I'd say probably each of those is worthy of a day. So a long-weekend thing would be perfect, maybe stretchable up to a week but probably not really any longer than that, unless you put another destination into the mix (like, say, go on over to Savannah for a few days and so on).