You're talking about the half-life of the molecule in the bloodstream, which is different from the half-life of the intensity of the effects. Serotonin receptors begin to downregulate as soon as the effects start, causing tolerance which lasts (to some extent) for several weeks. Taking a massive dose of LSD, or any other classical psychedelic for that matter, wouldn't significantly extend the trip. The brain just refuses to stay in that state for long.