Acidity is a logarithmic not a linear progression because bacteria theoretically double their population every 20 minutes. Every bacteria has a pH curve that predicts the rate of acid production, and for the first 2-3 hours (or more), there are not gigantic drops in pH. Depending on the cheese, the bacteria used, and the amount of starter used, that bacterial population and associated acid production "kicks in" from 3 to 5 hours into a make. You have complete control over over acidity at various steps along the way and may have to pitch, salt, and press when the cheese is ready, not because a recipe dictates a certain time.