The recommended amount is 400 mg a day.  I sometimes take in twice that much.  Helps a lot of stuff, i read somewhere magnesium is perhaps one of the top things needed for bodily function, just behind oxygen, water, and stuff like that.

YOUR CRAVING       WHAT YOU MAY NEED      WHAT TO EAT INSTEAD
CHOCOLATE        MAGNESIUM              NUTS, SEEDS,  LEGUMES, FRUIT

Many of us need way more than 400 mg a day. More important than the total amount, for many people, is the ratio of magnesium to calcium. Official recommendations are to have twice as much calcium as magnesium, but we all have unique needs. Some people need equal amounts of both minerals, or a 4:3 ratio instead of a 2:1 ratio; some people even need more magnesium than calcium (at least for short term to correct deficiencies.)

If you're taking a cal/mag supplement, and take more pills to increase your magnesium, it may have no effect due to the calcium content of the pills.

also good info in above post on relationship of Vit D and magnesium. Good to keep in mind for those taking high amounts of vit D

Yup, all these nutrients work together in a complex relationship. Adding just one can sometimes cause more problems than it solves.