A thumbprint on the lower end of the scale would be atleast several grams of crystal LSD... we're talking tens of thousands of doses or even more, perhaps much more. Anything less wouldn't be a "thumbprint", you could eat 100 tabs and thats not even touching the edge of the realms of a thumbprint, by a loooong way too.
This isn't something a person just decides to do. Apparently thumbprints were used as a sort of 'rite of passage' (only if chosen and seen as worthy) to join one of the acid "families". A dose of that unimaginable quantity would kick in almost instantly. Apparently you will literally immediately have your head in the toilet after dosing, violently throwing up, then go into a coma-type state for multiple days. People talk of coming into direct contact with 'god' and other mindblowing things, mindblowing being a pathetic understatement of a word regarding something like this.
You would not be the same person after something like that, for better or for worse I do not know.
Probably already been said in this thread but theres an interesting thread on shroomery.org about thumbprints.
Hrmm. Its my understanding that LSD has a ceiling dose, where the receptors are saturated and dosing above this will only serve to increase the duration?
1200mics rings a bell, but yeah regardless of how much more effects you get from doing such an insane dose, the duration would be what makes the experience special.