Sammy neal · taken for 5 to 10 years january 9, 2014. Sometimes, this can occur in as little as half an hour. Milk Of Magnesia Oral Uses Side Effects Interactions Pictures Warnings Dosing - Webmd Click to see full answer. How long for milk of magnesia to work for constipation . Sometimes bowel movements can occur in as little as half an hour. It may take up to 72 hours for laxative action to occur. You can expect to have a bowel movement within 6 hours of taking milk of magnesia. Sometimes, this can occur in as little as half an hour. Milk of magnesia usually relieves constipation within 6 hours of taking it. People with constipation may find that drinking between half a cup and 1 cup of prune juice in the morning helps stimulate digestion. It will help ease the release. You can expect to have a bowel movement within six hours. It's a saline laxative that's used in patients that are about to undergo a really long surgery, but can be used to treat temporar...