Anecdotal but whenever I’ve been asked for this guidance (having been around the block, but not a shaman by any means) I ask about the person’s dreams. Do they usually remember them, how intense are they? More intense = start with lower dose (maybe .5g). Less so maybe start with 1 or 1.5g. Of course this is subjective and relative but a rule of thumb that has worked reasonably well.

For frequency, that depends on a variety of factors I suppose.