How quickly can bird droppings damage car paint?

There is no universal safe time, because damage depends on temperature, paint condition, the dropping's chemistry and sunlight. On a hot panel, etching can begin much faster than many drivers expect, so bird droppings should be softened and removed as soon as practical rather than left until the next wash. Soak the dropping with a lubricating spray or water, let it soften, then lift it off without scrubbing.