If they continue traveling together even after they reach the Demon Kings castle, then their relationship may very well span 95% of Fern's life, while still not even reaching 1/10th of Frieren's. Heck, taking into consideration the 1000 years flashback from the previous chapter, where Frieren looked only slightly younger, I would say her lifespan is no less than 4000-5000 years, so even Fern's whole life wouldn't reach 1/50th.