Well the term heavy user would to me indicate someone that uses LSD frequently? If so then they could be taking large doses of good LSD.
My experience and several other people I have talked to on the matter agrees with this chart describing tolerance.
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For example, Friday nite drop a hit and fry my balls off. Sunday nite I drop a hit and nothing. Same sheet, stored in the freezer in a vacuum seal bag so the acid is still good but my tolerance at 2 days says I would need to take a dosage of 225%, so instead of one hit I would have to take 2.25 hits to get the same fry. At one week the tolerance is still at 120 percent.
So lets say we have a heavy user and he got some quality paper. Starts out with 2 hits on Friday. Wants to fry on Sunday again and he takes 4.5 hits (225% @ 2 days). Then it's say Friday and it's fry time lol, dude is now at 5 days so 135% tolerance so he drops 6 hits. Now it's Sunday and Fry time (again lol), another 225% bumperino so thats 13.5 hits all to get the same fry he got on 2 hits... another week of that pattern and on Sunday he is at 40 hits. Thusly over time an incredible tolerance can be built up. Explains how Jimi Hendrix was allegedly able to drench his headband in LSD and perform on stage. I have lived it and the chart is pretty spot on for me. My buddies and I would call big doses landing strips. It is way better of course to wait at least a month between frys but when I was younger you couldn't convince me of that lol.