It depends on personal taste and to some degree, on its ABV.
Most people suggest at least a few weeks, if not a few months, some people even manage to let theirs sit for years (not many can wait that long) but I'm not sure it'd generally benefit from ageing that long.
Ciders get better with age because harsher flavours tend to mellow out with time.
Hard Cider is drinkable as soon as it's done fermenting. But depending on your taste, let it sit. Maybe try samples, after week, 2 weeks, 1 month, etc, and see what you enjoy the most.