How Long Can Kitten Replacement Milk Be Left Out?

I have a female kitten who has been dry feeding for about 4 weeks now. She is getting 2cc every other day via syringe, but she’s still very hungry. Is there any way to get her used to the bottle? And how long can milk be left out for a cat that isn’t nursing? I’ve already tried putting it in her water dish and leaving it out overnight, but the next morning she’d drink it all up then stop eating again.