"Days"?!? Hard pass. My experiments with shrooms in college was fun, but I couldn't deal with the 8-12 hours of hallucinating.
I knew personally someone whose trips usually took 48 hours. Unusually long, yes, and really exhausting.
Admittedly i wanted to try both but back then I didn't have opportunity, and now the mindspace is just not too.... Right for the risks.
Maybe when I'm too old to worry and it's too late to care :)
Also, 12 hours is definitely not short of LSD. I'd say it's the standard duration, with the peak lasting 7 hours. Longer trips can happen, at least to some people, but the default assumption should be about half a day.