The Tolerable Upper Intake Level for magnesium supplements is 350mg/day for anyone over 9 years of age. There is no UL for dietary intake of magnesium; only for magnesium supplements. While doctors may prescribe higher doses to treat specific medical problems, I'd be weary of taking a higher dose without my doctor's consent.
I take a 250mg tablet with some pomegranate juice in the morning for my anxiety related prehypertension. This gives me some beta blocker/ACE inhibitor benefit without the side effects. While not a substitute for treatment, it may help me avoid the need for a while, combined with some diet and exercise of course. We'll see in 3 months when I go in for a follow-up.