If someone is interested, Ketamine is not a psychedelic but a dissociative, it has a different effect though some parts are similar (both classes are hallucinogens). Though both psychedelics and Ketamine can be used to help with depression.

Re how many times you'd have to take them: I have found that the effects (on my mood, not the high itself) from mushrooms last for a month minimum, most of the time even a bit longer. But as far as I'm aware, the idea would not be to take them say once a month. Instead, it's kind of like a reset switch, you take it once and then you're a lot less depressed. In that time where that effect holds on, you can now work on things that normally you can't due to the depression. Say someone is far too depressed normally to start a workout routine and we know physical activity helps with depression. If they have a month or even a bit more to start working out say 5 times a week, then this in turn will have an effect even after the initial effect from the psychedelics have worn off and hopefully the person does not slip back into a depression that's as deep. Plus there's a potential for having insights that you normally don't have which if you can hold on to those insights might help in the long run.