When Romeo and Juliet first speak of the possibility of getting married, Juliet asks Romeo at what time she should send a messenger to him to learn of the arrangements he's made for the wedding. He replies, "By the hour of nine" (2.3.182). Later, Juliet sits and wonders why it is taking the Nurse so long to return to her with Romeo's answer. She says to herself:

The clock struck nine when I did send the Nurse.
In half an hour she promised to return. (2.5.1-2)

Despite the Nurse's promise to return quickly, she has been gone for "three long hours," and Juliet is becoming incredibly impatient to learn what her future will be with Romeo (2.5.11). Juliet laments that her Nurse is so old and slow when she, herself, is so anxious and in need of some speed. Juliet wants to know if Romeo has made arrangements for their marriage, and she does not want to wait any longer.

Juliet sent the Nurse at nine o'clock in the morning to meet with Romeo to find out the time and place when their marriage would be consummated.  Juliet begins to panic because the Nurse doesn't return until 12:00 P.M. 

In Act II Scene 5 Juliet becomes impatient with the Nurse since she set him at 9:00 A.M. when the clock sounded and she has been gone 3 hours. The Nurse is very talkative and was probably clacking like a hen with all her friends.

The Nurse left at nine o’clock and it is twelve by the time she returns (Act II, scene 5).