For many modern web based businesses that practice continuous deployment and test-driven development, they may not have an explicit validation period where SQA and final validation is the focus. They could run entirely off of an agile software development process and push sprint outcomes directly to production (as they can always roll back a change if needed)
For complex, high availability systems with service level agreements (i.e. systems that are mission critical that need to stay up), a validation period is necessary to ensure that the product release is well characterized and thoroughly tested through major workflows (both positive and negative) before the product is released to end users. In this case a 2:1 ratio between development time and SQA time is prudent.
This also goes for software that are installed on custom hardware (especially if the hardware has the potential of causing some type of damage either to itself or its environment or operators).