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It is common run a starter on a stir plate for 24 to 72 hours. How long it too long depends several factors such as amount of yeast, starter volume, size of the flask (I assume a flask but vessel too) and the vortex rate. Also, is it covered, open, sealed, air injected? Generally, 48 hours on a typical starter is enough for the yeast. Keep in mind the amount of oxygen is low on open air versus pure O2 which is why a stir-plate helps quickly propagate the yeast.We probably all know oxygen is OK at certain points and BAD at other points in brewing. What about a stir plate? How long is a good/safe time to let the stir plate run without introducing unwanted oxygen? I ask as, basically, a starter is a smaller volume brew.
Oxygen is bad for fermentation once yeast go anerobic and kick in the fermentation process. Some high ABV beers could have small additions of oxygen in the first 36 or so hours while the yeast are still growing in population and getting strong, healthy cell walls. Beersmith talks a little about oxygen here Beersmith Oxygen
I could not find a perfect reference (Braukaiser is a good place to read up though). White Labs suggest 24-48 hours White Labs. This is about propagation, not too much oxygen though.
Thus, a good, safe time is 24-48 hours to propagate the yeast.