How long do LSD last in your body?

It really depends on what you mean. If for purposes of drug/toxicology screening, then all but the most sophisticated tests won't detect it anyway. The quantities involved in an active dose (200microgrammes approx.) are beneath detectable levels.

LSD is processed and out of your system in a matter of days as a general rule. "Flashbacks" are due, in my opinion, to permanent alterations in perception caused by highly psychoactive drugs being triggered by stimulii that "remind" the subconscious of the initial trip. As an ex-heavy user of various hallucinogens, this is what I feel causes my own occasional episodes of heightened perception.

I will neither condone nor condemn another's drug use, however I have this to say- BE CAREFUL WITH YOUR MIND. YOU ONLY HAVE ONE! And it's a beautiful, incredibly complex thing.