The main ingredients in McCormick Broiled Steak Seasoning Blend
are salt and MSG, which should last forever as long as they're kept dry. Other ingredients are black pepper, onion, garlic, and unspecified spices. Those will deteriorate over time, especially if exposed to heat, light, or moisture. I therefore suggest keeping the stuff in the freezer in an airtight plastic bag or container.
All herbs and spices should be kept in a dry, dark, place, such as a cabinet or drawer, to prevent deterioration and extend their shelf life. Although they generally don't need to be refrigerated, since you have such a huge quantity of this blend, I think the freezer would help to preserve it as long as possible.