We had good luck with our starts this year and ended up with a really nice harvest. You want to harvest sweet potatoes before the frost. They like cool nights - and I hear tell that they get sweeter with cooler weather, but any frost will nip the vines and once the vines are frozen, they'll start to rot and then the potatoes rot quickly, too.
So, cover them for light frosts, and harvest before a hard frost. If you happen to get caught by a surprise freeze, then take any frozen vines off right away. That will prevent rot from getting to the roots for a while.
We planted our starts well after the last frost date in May, in a 4' x 8' raised bed full of sand and chicken dirt. The potatoes loved it and we had gorgeous vines all summer.
What you can see is just the beginning of what's under there. There are potatoes in the center of that cluster that go down quite a bit deeper.
Lift them out carefully - the potatoes crack easily if you twist them.
Look how big some of these clumps got! That's Eric's hand lifting this clump out.
Just brush the soil off and let them dry.
We brushed them off again, then loaded them in boxes and put them in the back room to sit until we eat them.