Every grower has it own way to determine when to harvest. Watermelons reach maturity approximately 45 days after blooming. Time to harvest is when the tendril nearest the melon is wilting and the ground spot has turned from white to yellow. Fruits have hollow and dull sound when thumped. Ripened fruits are shiny with nice coloration. Immature WM have turgid tendrils. Completely wilted tendril and pale color can be indicative of over maturity.   Watermelons unlike sweet melons can't ripe after picking. Sugar content could reach more than 12% level and minimum is 10%. Dry matter (Brix) at 10% level. Harvest WM in the afternoon when they have lower turgor, so there is  less chance of cracking. Do not wash the fruits with strong water flow as it will remove wax layer and reduce shelf life.