Quote from St Alphonsus Liguori from his book, ‘The Glories of Mary’ : 

“We can salute the Mother of God with a ‘Hail Mary’ every time we hear the clock strike. Blessed Alphonsus Rodriguez saluted her every hour; and at night, if the hour had passed without his doing so, Angels awoke him, that he might not omit this devotion.” 

Further reading can be found at: http://www.catholictradition.org/Mary/mary23.htm