People obviously change over time. How long will your results be useful to me?
Actually, people don't change over time. Yes, many things about you will change: your jobs, your relationships, your ideas, your opinions, your beliefs, your habits, your body, etc. But your core personhood never fundamentally changes. You remain who you are. And your giftedness will remain stable throughout your life.
As a result, once your giftedness is brought to light, that information can be used to great advantage the rest of your life. It never really needs to be "updated."
Having said that, it's worth mentioning that it can be advantageous to repeat the giftedness assessment process as you grow older and have more experiences in life. Those new activities can provide more data with which to describe your giftedness. The result is not some entirely new description, but rather an embellishment of the original description that has greater precision and nuance.