From my experience, it took about a month of 150mg magnesium supplementation per day to see effects, two months for my insomnia to go away entirely.

Doing a quick search for magnesium-rich foods, it seems like it would take either an unsustainable focus on eating these foods to see results on a similar timeline or an extremely prolonged timeline to see similar results.

That said, I am not a doctor or nutritionist. But my instinct is that you can get so far in the hole with a lifetime of magnesium deficiency that supplementation is by far the most time- and lifestyle-efficient to digging yourself out. Whether or not switching to magnesium-rich foods can sustain you being at correct levels will end up depending on what the size of the deficit was in the first place, but is probably worth a try.