How long magnesium stays in your body depends on the type of magnesium it absorbs. In general, most magnesium leaves the body within two to four hours of taking an oral supplement. Studies indicate that the body typically returns to normal levels within six hours of taking magnesium.  Your body stores the majority of its magnesium in bones and tissues rather than your bloodstream. The remaining magnesium is absorbed, processed by your kidneys, and released through urine within about one day. This timeline will vary by person to person depending on kidney health and overall magnesium levels.