Every dog is different, as will every answer. For dogs with fur needing regular haircuts, and if the hair is long and you feel knots and tangles in their coat, groom as soon as possible.
If you cannot see their eyes, this is another sign they need grooming. You do not want their fur length to be so fluffy that you cannot see any features on the dog (legs, eyes, nose, tail, etc.).
If a dog sheds a lot, you will notice a lot of hair falling out regularly, meaning that grooming is required.