How long does it take gnats to die once its food source is removed?

Question

I found where the gnats were breeding in my house earlier today and disposed of the source (rotting produce). I've seen a couple still flying around this evening. How long will it take the few remaining to die out? Should I spray something this early or wait until they die on their own?

Answer 1

They will be dead by morning.. they only live 24 hours anyway.