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Many have seen the MB Raines article on Yeast propogation. In the chart below, it is suggested that the optimum time for pitching is 18 hours after making the starter.

JZ agrees:

Wyeast says starters reach their maximum cell density within 12-18 hours, which sounds about right to me. Some sources suggest that maximum cell densities are not achieved until 24 hours and others as much as 36, but the return on waiting that long is minimal. Let’s just say that the bulk of the yeast growth is done by 12-18 hours. I like to pitch starters while they’re still very active and as soon as the bulk of reproduction is finished, usually within 8 to 18 hours. This is really convenient, because I can make a starter the morning of the brew day or the night before the brew day and it is ready to go by the time the batch of wort is ready. There is no need to make a starter a week in advance, because I pitch the whole starter, liquid and all (up to a certain size of starter). Yes, you can wait longer and completely ferment it out so you don’t have to pitch the liquid, but if you’re going to do that, you should use a larger starter and allow the fermentation to go complete cycle over several days, chill, decant the beer and pitch just the yeast. If you’re making a smaller starter, it is better to just pitch the entire active starter within about 6 to 12 hours of pitching the yeast into the starter.

Anyway, here’s a couple of pics of a situation I found to be very interesting about a year ago, and I wouldn’t have thought anything if I hadn’t had two starters (begun at the same time) going side by side. These 2 starters were created at the same time, 7pm on Saturday night. I didn’t take a picture at the very beginning, but the color of the 2 starters initially was the same as the darker of the starters in the first pic. (the one on the right).

The first pic was taken at noon on Sunday (17 hours later). You can see that the 4L starter on the left appears to have reached high krausen and maximum cell density (indicated by the foam on top and light, creamy color of the wort… looking very closely at the time, its as if you can see the billions of yeast cells swirling around), but the other 2L starter is still the same color as when the yeast tube was initially piched at 7pm the night before.

The 2nd pic was taken at 9pm on Sunday (26 hours after initial pitch). The 2L starter now appears to have reached high krausen and maximum cell density. I think its just interesting to note how the 2 starters behaved differently. The 4L starter here that maxed quickly was a WLP570 Belgian Golden Ale yeast with a use by date of 5/31/06. The 2L starter that took longer was a WLP023 Burton Ale yeast with a use by date of 6/30/06.