2-3 grams. If your body is prone to nausea or vomiting, stick to 2 grams or under. If you have opioid experience/tolerance and know your body can handle it, you can go higher. Mitragynine and 7-hydroxymitragynine, the main active compounds, act similarly to an opioid, albeit much milder (as in you can't get ridiculously high off of it, but beginners will feel light euphoria), less addictive, and safer. If you know anything about opioids, you know they can cause nausea and vomiting, and so kratom can do the same thing.
One interesting note about kratom dosing is that lower doses (5 grams or under) produce a stimulant effect, while higher doses will induce an opioid-like sedation and relaxation. Since I pretty much use it as like a coffee to help me with productivity and social situations, I have stuck to consuming a stimulating dose of 3-4 grams every time I take it, even after a year of daily use. One of the best things about kratom is that tolerance does not build much, and unlike traditional opioids and most drugs, you can take the same dose again and again and still feel the same effects. I seriously can't rave enough about this substance, it is a life changer.