Juliet sent the nurse to meet Romeo at 9:00 A.M. and she didn't return till 12:00 P.M.  This was after she also bought things in the market and had to walk.  The time was not strictly spent on finding Romeo, arranging the marriage and going home.  She carried out ther daily committments besides her meeting.

It takes the Nurse three hours to get back to Juliet. Waiting impatiently at home, Juliet says,

 Of this day's journey, and from nine till twelve
 Is three long hours, yet she's not back.
 Had she affections and warm youthful blood,
 She'd be as swift in motion as a ball.

Juliet felt the Nurse should have been back in half an hour, but three hours is much too long. She says the Nurse would understand that and return sooner if she were younger and remembered what it's like to be in love.