I've tried now and youtheory 400-500 mg. Both seems to work for me. It takes about an hour to take effect, and lasts for another few hours. I experimented with it quite a bit when I was interviewing. I did lot of reasearch, and determined that taking it intermittently is probably the best way to stave of potential side effects. Again I strongly caution to read as many accounts from people that've taken it as possible. Many reported positive effects, but lot of them also reported wide variety of side effects that shows up after taking it regularly.