I've done both...pitch at full activity and once the fermentation was complete, chilled and decanted. I've usually found that 12-18 hours is good for peak of fermentation. The starter will go from the starting amber color to a creamy tan-ish color. I usually have minimal krausen (compared to a full 5 gal batch) and the color change is a good indication. Once swirled (no stir plate) to aerate, there is a pretty good head to the starter as well.