Jesus told an important parable about the persistent widow as a model of prayer in Luke 18:1-8, especially V1 -
Then Jesus told his disciples a parable to show them that they should always pray and not give up.
Now, it is well-known that women in Bible times, from at least the time of Sarah, Rebekah, Rachel, Hannah and Manoah's wife, to Elizabeth longed to have children and prayed often to have this divine blessing.
Based on this context, I would reasonably assume that Zechariah and his wife Elizabeth and prayed many, many times for a child. Just how often they prayed we are not told, but I am certain it was much, much more than once.