Consider the research and development cost of a machine. Let's say in 2013, a prototype was manufactured. Then in 2014, a patent application was filed. On that same year, validation continued. Significant developments occurred until 2018. On 2019, the research was declared complete and a success.
Does the useful life begin on 2013 when the prototype was made, or on 2014 when the patent was filed, or on 2019 when the machine was perfected?