How big should a watermelon vine be before watermelons grow please ?

I planted about 7 small plants a few months ago. The vines are about
2m long now. They flower but that is all they do. The leaves on the
vines are only about 3 inches long.

Is there a site where they have images on how big the vine gets to
give me an idea on how much growing the plant has to do before I get
watermelons growing please.