Mine was pretty spontaneous. I was studying psychedelics at the time (just because they’re fascinating) but I’ve never done any before or since.

It was… hard to describe. It lasted several days at least, but my sense of time was greatly altered and it’s hard to say how long exactly. I remember feeling like my mind wasn’t fighting against itself the way it usually did. It felt like everything I did, my whole brain was all working/pulling in the same direction. Pretty much all I wanted to do was meditate for hours on end, and doing so was a wild experience with some very interesting visuals. I also came to some revelations about the nature of reality. (Though looking back, those revelations were the logical conclusion of several beliefs I had held before this experience. I think this experience just brought those multiple unrelated beliefs together and crystalized them into one cohesive worldview.) I did experience some synesthesia during the experience as well. The kind wherein seeing somebody else experience something, you feel it in your own body. I was watching a dancer on TV and feeling the proprioceptive feelings I imagined she was feeling.

Edit: I should add that it never really “ended.” It tapered off over time until I was (in some ways) back to normal, but I couldn’t identify really when I was back to normal. It was more like asymptotically approaching normal. And, I’ll also say that in other ways, I’m still changed by that experience. And only for the better.