Cultivation as an Immortal was an endeavor of centuries if not millennia. In fact, even that may not necessarily be enough to allow an immortal to move between the levels of a single Immortal realm. One simple example of this was Ragnar.
He was one of Jotun's finest generals, and was considered a rising star because he had been able to consistently raise his cultivation level each century despite being so occupied by his responsibilities.
The true difficulty of rising in the Immortal realm was that, unlike previous cultivation which was, in essence, the stable accumulation of spirit energy, cultivation as an immortal was mostly focused on comprehension. Whether it was comprehension of laws, of their own tenets, of the various complexities of the countless energies in the universe, any of them would work. In fact, the possibilities were far greater than that, as was evident from Lex's own cultivation.
