There is no simple threshold, which is why changes are raised rather than measured internally. The question is whether the work still requires the special knowledge described and whether the described conditions still hold. Drift toward general project delivery, or toward supervision, moves the assignment away from the basis it was approved on.
Ask what the filing said this person would do
Compare the current work to the original description honestly, including the parts that no longer happen. If the specialised element has become a small share of the week, that is a fact for counsel, not something to argue away internally. Approval described a set of circumstances; it did not license whatever the project later became.
Raising it early preserves options that disappear once the change is entrenched. It is worth restating the condition the assignment was approved against, because drift is measured from it. The applicant must have been employed continuously for one year within the three preceding years by a qualifying entity abroad, in a managerial, executive or specialized knowledge capacity, and the United States role relied on specialized knowledge as defined.
Neither of those conditions is retested by the passage of time, but the second describes the work, and work changes. Where the specialised element has thinned, the practical question for counsel is whether the current assignment is still the one that was approved or has become something else that needs its own analysis.