When does Hamlet intrude into Ophelia's private quarters?
When Polonius and his daughter Ophelia speak in Act II, Scene 1, Ophelia describes Hamlet as having his shirt hanging open and and stockings crumpled down at his ankles, a clear sign to the Elizabethan audience who recognize this attire as that of the "anguished lover." Indeed, there was a belief in love sickness. As she speaks with her father, Polonius, Ophelia tells him, also,
And with a look so piteous in purportAs if he had been loosèd out of hellTo speak of horrors—he comes before me. (2.1.82-84)
Ophelia is scared from this meeting because suddenly she sees Hamlet and he goes up to her and shakes her three times. His appearance is distraught and once she tells Polonius what happens, she starts to believe that this was the cause of Hamlet's love for Ophelia.
This is an interesting sequence. In 2.1 as Reynaldo is leaving his meeting with Polonius, Ophelia enters. Ophelia enters distraught, "affrighted" and in urgency to speak with Polonius. Her appearance and her motivation are driven by having just come from her closet where she had the encounter with Hamlet. I would say that Ophelia's encounter with Hamlet in her closet occurs at the same time Polonius is plotting with Reynaldo. With two events occurring simultaneously one is expressed mimetically (Polonius/Reynaldo) and the other diegetically (Hamlet/Ophelia as narrated by Ophelia to Polonius). This doubling up isn't isolated in the play. Most noted is Gertrude's narration of Ophelia's death in 4.7 the event of which occurs as Claudius plots with Laertes to kill Hamlet.