How should we know "when" refers to something durative or not?
"When" is a subordinating conjunction. It is used to introduce subordinate clauses. Clauses have subject phrases and verb phrases. If the verb phrase is durative, then the "when"-clause itself refers to something durative.
Suppose I say, "I will read a novel when I fly from California to Virginia." The flight will take longer than an instant; it will actually take five hours. Thus, the verb phrase and the "when"-clause refer to something durative.