The best example in Hamlet of the theme "love wanes with time" is in Act III, scene i in which Hamlet and Ophelia have their famous "I did love you once" speech. Ophelia approaches Hamlet, interrupting him in the middle of his "To be or not to be" soliloquy. She is proceeding on orders of her father Polonius and Claudius, the King. She is to try to snare Hamlet into revealing the origin of his inexplicable behavior that to some observers looks like madness. Ophelia has been instructed to return to Hamlet the "remembrances," gifts, he had bestowed on her with "words of so sweet breath."
First Hamlet doesn't recognize what she means by "remembrances" that she had "longed long to re-deliver" and states he never gave her "aught." The first language of love that wanes with time is Ophelia's claim that she "longed long to re-deliver" the gifts of love Hamlet had given her. The next statement of love that has waned with time is again Ophelia's and she says, "Rich gifts wax poor when givers prove unkind."
Hamlet is stunned and replies with the statement that if she were honest and fair, her honesty could now not permit the entry of "discourse," or companionship, with her beauty, implying that her beauty has deceived and betrayed him. Hamlet goes on to say that beauty, if allowed to mingle with honesty, will degrade honesty, which he had thought a "paradox, but now the / time gives proof. I did love you once." Love has waned with time.