Over time, a wizard uses a particular evocation often enough that ... its use becomes reflexive. When this happens, the spell is considered to be a rote spell.
(YS, p. 257; emphasis mine)
How often is "often enough" to allow a wizard to change their rote spells? When a certain type of milestone is reached? When that exact evocation is cast a certain number of times in-game? Does it matter if the spell was not safely controlled? Does the answer change if the wizard switches one rote for another, instead of adding a new rote?
I have already checked every instance of "rote" in Your Story, but none of them were conclusive. I would prefer a RAW answer over homebrew, but experience-backed homebrew works if no RAW answer exists.