How can a woman know if the grape juice or vinegar she is preparing for her home has turned into an intoxicant (khamr), and is there a specific period of time during which grapes turn into permissible (halal) juice or permissible (halal) vinegar?

There is no harm in drinking grape juice as long as it does not reach the point of intoxication, for it is among the good things that Allah has permitted for His servants. However, if this juice is left until it changes, such as by becoming potent, or foaming, or boiling, then it is forbidden to consume it, because at that point it becomes an intoxicant or is likely to become one. There is no specific duration for the juice to change, as this change varies with times, places, and temperatures. The Prophet, peace and blessings be upon him, had forbidden steeping (dates or raisins) except in a waterskin, then he permitted it in any vessel as long as it did not become intoxicating.