PlantLife Online

Answer

It depends on the variety, generally between 70 and 85 days from planting. The three main types are as follows: Early Season 70-75 days, Main Season 80-85 days and Seedless 80-85 days. Determining when to harvest watermelon can be difficult however there are a few things to look for: (1) The skin of the watermelon becomes dull and tough. (2) The side on the soil is light-green, yellow or creamy white. (3) Feel the skin for slightly raised ridges. (4) The tendrils near the fruit stem will dry and turn brown.

Many people stand by the thumping method using their finger, I feel it's very hard to describe what the perfectly ripe watermelon sounds like. It is to subjective in my opinion. 

Here are a few common varieties:
Early Season:
Golden Crown (red flesh, green skin; skin turns yellow when ripe)
Sugar Baby (red flesh, 6 to 10 pounds)
Yellow Baby (hybrid-yellow flesh, 6 to 10 pounds)
Yellow Doll (hybrid-yellow flesh, 6 to 10 pounds)
Main Season:
Charleston Gray (red, 20 to 25 pounds)
Crimson Sweet (red, 20 to 25 pounds)
Madera (hybrid-red, 14 to 22 pounds)
Sunny's Pride (hybrid-red, 20 to 22 pounds)
Sweet Favorite (hybrid-red, 20 pounds).
Seedless:
Cotton Candy (red, 15 to 20 pounds)
Crimson Trio (red, 14 to 16 pounds)
Honey Heart (yellow flesh, 8 to 10 pounds)
Nova (red, 15 to 17 pounds)
Queen of Hearts (red, 12 to 16 pounds)
Tiffany (red, 14 to 22 pounds).

http://plantlifeonline.net/plants/fruits/watermelon