You don't want to trip for days. Even 8 hours can become quite tedious without it being a bad trip, if things go sideways or it starts out bad then days would be horrible. As a comment [1] in a recent thread I talked some about my past psychotropic use as a youth, including some of the bad experiences during trips that would last only minutes but seriously disturb me, being in that state for even 24 hours could spiral fast and just be horrible.
[1] https://news.ycombinator.com/item?id=22225541