Within 24 hours. If your beer is in a bucket, take a peek by prying open the lid, after removing the airlock. If bubbles are forming on the surface, it's fine. If 24 hours after pitching the yeast, the wort looks still (no bubbles or foam) you need to add another packet of yeast as soon as possible.
As long as it is still bubbling it is still converting sugar to alcohol. Take a gravity reading and see how close you are to expected gravity and go from there. Could be a couple of weeks or more.
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