I've never seen this clearly defined. At the root, criteria for engineering come from the intended uses. If the product is meant to last for 5 years and you des

I've never seen this clearly defined. At the root, criteria for engineering come from the intended uses. If the product is meant to last for 5 years and you design the product for 50 years, you've probably over engineered. If the product is meant to last for 5 years and you've designed for 6 years, you MAY have under engineered, given a statistical variance for failures over time (e.g too many failures happen at 4 years instead of 6) So, an academic answer, it depends :)