How long to fix Mg deficiency?

It took my plant about 3 days. The leaves that got too yellow or had burnt spots most likely won't fully recover though and your plant may want to use their nutrients and drop them. But do not worry, as long as it is just some true leaves, your plant will be perfectly fine with it. Please make sure your ph is within the right range for the grow stage your plant is in as it can be the reason of the deficiency!