There is a window, but then that window opens up again. What people mean by a window, is for a single dose. You have a certain amount of time to get that dose in for it to stack. If your taking another dose 5 min later your taking a new dose not adding to the previous one, most likely.
All of this is also pretty much unknown though. I've smoked blue lotus dmt joints and i was able to increase the effects with repetitive hits. My second or third hit of a particular experience was easily a full minute or more after the first hit and propelled me super far after already feeling threshold effects. Not sure what the mechanics are really, just know that with dmt you never really know lol.
"Energy flows where attention goes"
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