I've never used it, but found the following thru Google,; last sentance says 1/2 hour to six hours:
From: http://en.wikipedia.org/wiki/Magnesium_citrateMagnesium Ccitrate
A magnesium salt of citric acid, is a chemical agent used medicinally as a saline laxative and to completely empty the bowel prior to a major surgery or colonoscopy. It is available without a prescription, both as a generic brand or under the brand name Citromag or Citroma. It is also used as a magnesium supplement in pills. The magnesium content of magnesium citrate corresponds to about 11% by mass.Mechanism of action
Magnesium citrate works by attracting water through the tissues by a process known as osmosis. Once in the intestine, it can attract enough water into the intestine to induce defecation. The additional water helps to create more feces, which naturally stimulates bowel motility. This means it can also be used to treat rectal and colon problems. Magnesium citrate functions best on an empty stomach, and should always be followed with a full (eight ounce) glass of water or juice to help the magnesium citrate absorb properly and help prevent any complications. Magnesium citrate is generally not a harmful substance, but care should be taken by consulting a health-care professional if any adverse health problems are suspected or experienced.Use and dosage
The maximum Upper Tolerable Limit for magnesium in supplement form for adults is 350 mg per day of elemental magnesium according to the National Institutes of Health (NIH). In addition, according to the NIH, total dietary requirements for magnesium from all sources (i.e. food and supplements) is 320–420 mg of elemental magnesium per day, though there is no UTL for dietary Magnesium. As a laxative syrup with a concentration of 1.745 g of magnesium citrate per fl. oz, a typical dose for adults and children twelve years or older is between 7 and 10 US fluid ounces (210 and 300 ml; 7.3 and 10 imp fl oz), followed immediately with a full 8 US fluid ounces (240 ml; 8.3 imp fl oz) glass of water. Consuming an adult dose of 10 oz of laxative syrup (@ 1.745 g/oz) implies a consumption of 17.45 g of magnesium citrate in a single 10 oz dose resulting in a consumption of approximately 2.0 g of elemental magnesium per single dose. Given that this laxative dose contains five times the recommended nutritional dose for magnesium, caution should be taken to avoid prolonged usage (i.e. over five days) and to follow the manufacturer's instructions strictly. For children between three and twelve years of age, the typical dose is roughly half that, based on physician recommendation. Magnesium citrate is not recommended for use in children and infants two years of age or less.
Although less common, as a magnesium supplement the citrate form is sometimes used due to its increased bio-availability to other common pill forms, such as magnesium oxide. However, according to some studies magnesium gluconate may be more bio-available than magnesium citrate. Higher doses, up to 500 mg daily, have been used effectively in the prophylaxis of migraines, in combination with riboflavin (vitamin B2) 400 mg and, in some cases, a supplement of coenzyme Q10. Similar dosages apply when used as a supplement to help prevention of kidney stones.
Magnesium citrate, as a supplement in pill form, is useful for the prevention of kidney stones.Side effects
It is always important to correctly follow the prescribed doses; extreme magnesium overdose can result in serious complication such as slow heart beat, low blood pressure, nausea, drowsiness, etc. If severe enough, an overdose can even result in coma or death. However, a moderate overdose will be excreted through the kidneys, unless one suffers from serious kidney problems.
Magnesium citrate solutions generally PRODUCES BOWEL MOVEMENT IN ONE HALF TO SIX HOURS
[emphais by me]. Rectal bleeding or failure to have a bowel movement after use could be signs of a serious condition.