How important is the mix ratio when mixing epoxies?

Two pack products have been specifically developed to cure at a certain rate based on the correct chemistry. It is recommended when mixing two component products that the correct mixing ratio is adhered to (Epiglass Resin and Hardener is always mixed in a 4:1 ratio), as insufficient or additional hardener will result in an incorrectly cured product, which will affect the longevity of the product.