Good advice here. Most studies on ashwagandha use the KSM-66 extract, so it's the safer choice. I've seen 600 mg/day (300 mg twice a day) of KSM-66 used in several studies, but one study used 125 mg once a day and saw results (though better results with 250 mg once a day, and even better results with 250 mg twice a day).
Look at the different studies referenced here:
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Studies are organized by what was measured (for instance, "cortisol" and "testosterone"). So, depending on your goal, you should easily find appropriate studies.