Once every three months is the sweet spot for me. I've noted virtually no negative side effects from the use of psychedelics, even in full on immersive dosages and poly drug combinations, at this frequency.
I have had periods in which I tripped with greater frequency (once a week or twice a month at my most reckless), with notable negative side effects both during and after the trip. Chronic side effects were mostly increased anxiety and paranoia, though nothing I couldn't deal with using meditation, diet, exercise and a healthy dose of philosophical reading.
However, I began to see that it was foolish to use two mind expanding tools (psychedelics and meditation) against one another like that. If the psychs are causing negative effects that I then have to counteract with meditation, et al, I'm not really getting anywhere fast. At three months frequency on the psychedelics, I find that these potent chemicals can be fruitfully integrated into a holistic health project.
One thing I've continually noted is the paradoxical character of psychedelics in their deep profundity crossed with childishness. The more of my mind I give over to psychedelics, the more they ask for. They can be greedy and envious, and in particular I have found tryptamines to have a nasty habit of making me want to trip all the time and then kicking my ass for doing too much during the trip. Ime, Increased frequency complicates the possibility of integrating psychedelics and throws their place in the structure of a life project out of whack.
Psychedelic hallucinations and perceptual alterations are radically solipsistic and will remain that way until the insights derived from the ephemeral states are integrated by the experimenter into the intersubjective constitution of reality. The messages we receive from that realm tell us to treat ourselves and our fellow men, animals, plants, and all being in a more humble and respectful manner. You have to come out of the depths of your own mind to do that. Otherwise, psychedelics will not hesitate to steal back exactly what they promised to give us in the first place.