You probably wanna measure before the 2 week mark just to make sure that your beer is actually fermenting properly (has your airlock been bubbling?). If your gravity hasn't moved you may have dead yeast or fermentation, meaning bad stuff. A steady Gravity reading means your ready to bottle, any sooner and you'll get bottle bombs.
You can also use your gravity reading to figure out your alcohol content.
Estimated ABV: (Starting Gravity - Ending Gravity) x .129 = Your ABV%
ex. 1.043 (or 43) - 1.011 (or 11) = 32 x .129 = 4.12 % ABV
"May your glass be ever full. May the roof over your head be always strong. And may you be in heaven half an hour before the devil knows you're dead."
Primary:Triple Rye Red Ale
Bottles: Lights Out (#8) Belgian Trappist ale Surly Cynic PM clone, Sierra Red Rock IPA, St. Paul Porter, Pumpkin Ale, Carribou Slobber