The timing of winegrape harvest is determined by assessing the main characteristics that determine the resulting wine quality; soluble solid content (Brix˚), pH, and titratable acidity (TA). Experienced vineyardists also assess other flavor components and overall potential for further ripening in any given season.
Some good resources for information on how best to measure these parameters as well as what you should be aiming for can be found at:
- Cornell's 'Veraison to Harvest' newsletter
- 'How to Judge Grape Ripeness Before Harvest' - Ed Hellman, Texas A&M
For a good record-keeping tool to keep track of your harvest of different blocks or varieties, or grapes coming in from other vineyards, see this Grape Inspection Form developed by Tim Weigle of Cornell University.