You can harvest sweet potatoes as soon as they are large enough to use for a meal. I tend to wait until fall to harvest them. You will need to let them cure for a couple of weeks and let their starch turn into sugar for better flavor. To cure you will need close to 80°F. degrees at 90% humidity. After you can store them in a place with about 55° F. I am curing mind now but unless I can quickly set up an area to keep them I will be canning them in a couple of week. I am planning on keeping a few to make slips for next year instead of ordering them. They are getting more expensive every year.