Your seed will sprout. The germanation rate may be less but with that much seed, I don't think you will be lacking for little marigolds.
Most seed will stay viable for years. It remains more productive if stored in air tight containers in a cool place though.
Have you ever heard of the Anasazi Cave Bean? It was found in a cave in New Mexico sealed in a jar. It was 1500 years old and it germinated!
Daisy
