3 days.
The first 12 hours were spent in what I call "The Dark Phase" of the voyage: Visions of darkness, destruction, all the evils in the world, ranging from crime in NYC to crime in Africa to the media brainwashing the people.
Then the "purge" (throwing up) marked the transition into Day 2: "The Light Phase," which was filled with Divine Light and lightness, great feelings of peace, and visions of birds, beautiful insects, and also an inner journey I neglected to mention that seemed to be a glimpse into some futuristic/alien civilization.
Then some more horrific visions (of shooting games of old Europe, of myself as a rotting corpse) marked the transition into day 3: "The Empty Phase," which was essentially neutral. It was marked by a great feeling of zen-like emptiness, extreme rootedness in the present moment, and simplicity of mind.
The main experience lasts 2-3 days, but I would set aside at least a week for the experience with a few days beforehand to mentally prepare and fast (or at least eat lightly). The experience demands a LOT of time and space for integration. You'll be thinking about it for a long time, I guarantee it.