i've had both sweet williams and the annual bedding type stick around for a few years running. i have a barbatus cultivar i trasplanted from PA- this is at least its 3rd year (who knows how old up there) and 'sooty' is in its 2nd year. i don't haave any more of the bedding types right now, but have had them go to 3 years before. it seems to be highly variable, though- hard to count on it-it's rather like snapdragons in that you lose a few more each year. since most dianthus don't need tons of water, it may be worth plopping a few into your ground and see what they do. if they die, oh well, but they're gonna in the compost heap, so why not? i don't think they'd hold potted for a long while.

other than those annual ones, most of them form perennial clumps. i have a bunch of different perennial types growing- d. deltoides, armurensis, carthusianum, grationapolis, knapii, and some others i can't think of or don't know the latin for off the top. i really like dianthus, if you couldn't guess. :)

I bought a 4 inch pot of heart attack from PDN 3 years ago - my patch is >3 feet in diameter and foliage is healthy all year. I have had firewitch for about 10 years - it even handles partial shade pretty well.

I don't have an id on my green leaved perennial dianthus. The flowers are the same size as sweet william but not clustered and kind of a dark magenta color. It's about 8" high at the most and will make a large mat in just a few years. My mom has had it for very many years. I took some to one or both of last years swaps. It didn't make a very pretty potted plant- I think it's a good alternative to creeping phlox.

I have an unnamed dianthus too that I got from Lowe's several years ago -- that thing has survived everything, from drought to flooding.