Say that "Juan" has a patent with a legal life of ten years. However, because the patent applies to an extremely niche area with no competitors, he estimates that its actual useful life is thirteen years.
All the textbooks, tutorials, and so on that I have read over the years say to amortize patents according to their useful life. However, the justification they give is always that the useful life may be less than the legal life; they say nothing about when it is the other way around.
In this situation, do I go with the legal or useful lifetime of the patent?