Is inhaling more than 5 helium balloons bad for you?
It might make you light headed or possibly pass out, but it won't kill you.
Last longer than what? Helium balloons are not especially long-lived because diffusion is related to particle mass (lighter particles diffuse faster), and helium is the second-lightest possible gas. Helium balloons do last longer than hydrogen balloons, because hydrogen is the lightest possible gas.