It varies depending on the person and the type of magnesium, and its effect on you. Anywhere from 250-500mg is pretty standard. Up to 1000mg for those with any form of malabsorbtion/considerable deficiency or more severe symptoms they are trying to counteract with the supplementation. (Like Fibromayalgia). But if your body does not take a higher dose well and you get digestive issues from a higher dosage, a lower dose of the 250-500mg area could be better for you.

Magnesium OXIDE (60% elemental) is a laxative, not absorbed by human body as much.

Better forms of magnesium are TAURATE,
GLYCINATE, CITRATE (16% elemental), MALATE (6.5% elemental), ORATATE, CHLORIDE (12% elemental) in form of oil. Chelated magnesium is far better absorbed than magnesium OXIDE but cost more. Complementary medicine practitioners rely on chelated magnesium, such as magnesium GLYCINATE, TAURATE and ORTATE (magnesium plus amino acids) to treat serious cases of magnesium deficiency. Those kind of magnesium have less laxative effect on the intestines than magnesium citrate, so they are recommended if you tend to have loose stools.

OXIDE appears to have high amount of elemental magnesium. One 500 mg capsule of magnesium oxide contains 300 mg of elemental magnesium. But little of that amount is available to the body because it is not absorbed and therefore not biologically available. One recent study reported 4% absorption rate of magnesium oxide. This means 12 mg of 500 mg capsule are absorbed and 288 may stay in the intestines, acting like a laxative.

if your not trying to get the laxative effect, bioavailable low elemental magnesium is Better. Because the body can actually absorb more of that. Some supplements don't obviously specify which they are on the front, others say they are a bioavailable type but are actually mixed with magnesium oxide to make them cheaper to produce. You just want to pay attention to what is in thw supplements you choose.