What I've learned in my old weight training courses is that the soreness from actually injured muscle subsides in a few hours, but beginners should wait at least 24 hours after every workout to give time for the muscles to heal.
Soreness that lasts for 2 days or more is just accumulated lactic acid, and there is no danger in working right through the pain. After a while you will be able to tell which pain is which, but you should always be careful if you are a "slow healer" because you might injure your muscles more if you don't wait long enough to heal.
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