How long until cold crashing a starter?

I'm having trouble getting a clear answer on this.

When using a stir plate, how long should you wait until you cold crash it?
I can never seem to tell when a stir plate starter is finished. There's almost no krausen. How can you tell it's finished it's work and started building its reserves?

Then, I will cold crash, decant, and pitch to primary.

Later, I am also planning on using a 5l or 6l starter, crashing it and dividing up the cake and freezing them in glycerin as per this thread: https://www.homebrewtalk.com/f163/d...tarter-then-why-not-farm-yeast-freeze-269488/

Thanks to anyone who can help get a definitive answer!

I always get a Krausen so I wait for it to drop. Hydrometer will tell you the story or by observing the yeast slurry at the bottom.

You want to wait until they go dormant so they can store up glycogen. They need that when they wake back up. There is no definitive answer but defiantly let it go at least one day after FG. Brewing Science and Practice has a graph for a stirred 12P fermentation showing maximum glycogen at 60 hours.

It's going to depend a lot on the size and gravity of starter, number of viable cells pitched, temperature and even the strain.

24 hours total is long enough.

Okay, two votes for hydrometer, which leads to another question: how do you take a hydrometer reading of a stir plate starter without taking a whole bunch of good yeast with it? Put it back in the starter?? (wouldn't that raise the risk of an infection, especially for yeasts that are going to be split up, stored, revived, etc.?)

I'll defer to a microbiologist at Wyeast. He didn't specifically say how long was too long, but the way it sounds to me is that 24 hours is plenty. To me, in a starter, taking a gravity is a lot more trouble than it's worth.

[ame=http://www.youtube.com/watch?v=ng0Ib7n4DIA]Wyeast on Pitching Rates and Yeast Starters - YouTube[/ame]