When is My Starter Ready to Pitch
A yeast starter is ready to pitch anytime after it has attained high krausen (full activity), and for about a day or two after it has settled out, depending on the temperature. Colder conditions allow the yeast to be stored longer before pitching to a new wort. Yeast starters that have settled out and sat at room temperature for more than a couple days should be fed fresh wort and allowed to attain high krausen before pitching.
A key condition to this recommendation is that the composition of the starter wort and the main wort must be very similar if the starter is pitched at or near peak activity. Why? Because the yeast in the starter wort have produced a specific set of enzymes for that wort's sugar profile. If those yeast are then pitched to a different wort, with a different relative percentage of sugars, the yeast will be impaired and the fermentation may be affected. Kind of like trying to change boats in mid-stream. This is especially true for starter worts made from extract that includes refined sugars. Yeast that has been eating sucrose, glucose/dextrose, or fructose will quit making the enzyme that allows it to eat maltose - the main sugar of brewer's wort.
If you make your starter using a malt extract that includes refined sugar, it is better to wait until the yeast have finished fermenting and settled out before pitching to the main wort. Why? Because towards the end of fermentation, yeast build up their glycogen and trehalose reserves; kind of like a bear storing fat for the winter. Glycogen and trehalose are two carbohydrates that act as food reserves for the yeast cell. Yeast slowly feed off these reserves when other food is not present, and use this food extensively to fuel the synthesis of essential lipids, sterols, and unsaturated fatty acids when pitched to an oxygenated wort. (Yeast will rapidly deplete their glycogen reserves when exposed to oxygen.) While glycogen can be likened to the fat that a bear stores for winter, the other component, trehalose, acts more like the bear's heavy fur coat. Trehalose seems to get built up on both the inside and outside of the cell membrane, and is generally believed to make the membrane structure more robust and more resistant to environmental stresses. By allowing the yeast starter fermentation to go to completion, these reserves are built up, and upon pitching, the yeast starts out with a ready fuel supply and a clean slate to better adapt it to the new wort. As noted earlier, though, these same reserves are used by the yeast while in hibernation, so if the yeast are left too long before pitching, the reserves may be depleted and should be replenished with a fresh starter wort fermentation before use.