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But it can take 70-90 or more for potatoes to mature. This depends on the variety. The potatoes can carefully be harvested at 7-8 weeks for a early harvest of small potatoes. For full grown potatoes you would harvest in the fall when the leaves turn brown. But it can take 70-90 or more for potatoes to mature.
I'm not sure that I understand your question, but sweet potatoes will begin to develop around 30 to 45 days after planting; after about 95 days, they might be ready to harvest, but they can be left longer as long as there is no danger of frost.