WHEN WILL MELONS START TO FORM ON MY WATERMELON VINES?

I PLANTED A WATERMELON PLANT IN MY BACK YARD ABOUT 2 MONTHS AGO.THE VINES HAVE GROWN VERY LONG,LATCHING TO THE LAWN AS OT GROWS.I SEE YELLOW FLOWER BUDS ALL UP AND DOWN THE VINES.MY QUESTION IS WHEN WILL I SEE MELON BALLS STARTING TO FORM?.DO I NEED TO PRUNE THE VINES OR JUST CONTINUE TO LET THEM GROW ON?