Hi TransientC

This sounds like a beautiful experience you`ve had. I have not taken pharmahuasca myself, but I can relate a lot to the "gratitude"-state you describe. From time to time I have those trips that leave me loving every aspect of existence and accepting everything as it is.
In my view, true happiness has a lot to do with experiencing the whole spectrum and loosing attachements/letting go.
Therefore I try to not chase the happiness to hard and gripping myself to it. If I do so, I can get frustrated quite fast, because I judge myself at every emotion/event, which doesn`t approve with my desired image of happiness.
My experience tells me, that all feelings should have their room and happiness includes all kinds of emotions. I guess without the negatives and the sadness, I would forget what happiness means, cause there wouldn`t be anything to compare

.
This said, I am of the opinion everything that arises in us, has its right and its not possible to stay in one mindset all the time. More important than trying to be happy all the time, I`d say is transforming/channeling the "other" emotions into something useful.
To your other questions: "Do I need to do this often to keep it"? / "Will the effects of it stay"?
I think more important than the frequency of tripping, is how much you work with the message in your daily life...nothing wrong though with tripping regularly (in general), as long as one is aware that it`s not the substance which makes the difference, but you.
Concluding, I`d say how much you have to trip and how strong the positve effects last, is strongly connected with your willingness/effort of intergration!
I hope my post was not to far off
Take care
PlantPipeline