I've been taking high doses of magnesium and B vitamins for two months. Haven't noticed any measurable improvement. The efficacy likely depends on the underlying cause.
Magnesium glycinate - 100 mg nighty. I haven't noticed any difference with sleep or tinnitus. I've read it's good preventative along with NAC, so I took extra before dental work.
I took magnesium threonate for a couple years. Mainly to protect against oxidative stress. Threonate can cross the blood/brain barrier. I still worsened. Can't afford it now.