It only took a few days. I started sleeping better and having tons of energy during the day. If it hasn’t started working within the first week, I would wonder a few things:
What type of magnesium are you taking
What time of day are you taking it
Are you taking all 400 mg at once - if so, try taking 200 mg twice a day instead
Are you taking any other supplements
Have your stools become loose yet
Zinc can make your body absorb less magnesium. On the other hand, some minerals help boost absorption. Vitamin D and calcium can increase your ability to absorb magnesium. Vitamin D is usually quite low at this time of the year unless you are supplementing. I use a liquid vitamin d that I put on my face every morning before my skin care regimen. I take a calcium (“caltrate” which is calcium citrate) supplement as well but get most of my calcium from broccoli.
Also, the next time you have bloodwork done, ask to have your magnesium and vitamin d levels checked. That will give a good indicator of whether you are absorbing the magnesium and have enough vitamin d to get it into your cells. The optimal range of vitamin d is over 30 but the normal range is between 30 and 100. At 50, it starts to act as a hormone directing calcium into the places it needs to go.
Unfortunately, there is more to regulating fatigue than simply popping a single supplement. The magnesium needs to be absorbed by the cells to work properly. Keep at it. I hope it works out for you