I've read that you need a dose of 25mg/kg three times a day. Which would be 2000mg x 3 for me. But I couldn't afford this much right now, since only liquid is available in my country and it is a lot more expencive. So I took 1600 mg at once and then another 800mg after several hours. I haven't noticed any effect at all, except maybe getting quite sleepy. But it's 10pm and I've been studying all evening, so who knows...
I also ordered some pills from ebay. 50 pills x 600mg. But they will only come after two weeks.