It lasts roughly 3 hours, I have two caps of 40 mg left and I redose about every 2 hours because it takes an hour or so for the oral dose to kick in IME. Dosing every two hours the next dose comes up as the previous one tapers off, and you get a fairly consistent level (as best as I can tell at least) of stimulation. People staying up from days on end are either vaping heroic quantities and then twigging out on the aftereffects, or are doing spaced redoses throughout the time.
The cravings from this stuff are so, so weird. I did not notice a rush from either of the snufflopagus doses, and yet my brain is still urging me to try it again because this time I might get something. It's like I'm chasing a high that I haven't even felt and don't know if it actually exists. Resisting for now because I want duration more than anything and as I understand it snorting reduces that. I think I will try snorting once more, after my tolerance has fallen off over christmas, with 40 mg or so, and then that will actually be that unless it's like getting smashed in the face with a gold brick wrapped in a slice of lime. I really, really don't trust this stuff, but on the other hand the upsides, as best as I can notice them, are pretty good. It's just so... shifty and sneaky that you can't help but want to keep an eye on it before it tries something really underhanded.
Edit: As I was going for a piss the perfect analogy struck me. 3-FPM is like you go to a hospital, everything is brilliant white, scrubbed within an inch of its life, all the potted plants perfectly manicured, production line looking staff walking around with big, cold, dead smiles that can't help but creep you the fuck out because nothing is that perfect without being completely fake and artifical. And you know, if you get put under its odds even you wake up in a dumpster round the back with all your internal organs nicked and donald trump's mutant hairdo surgically grafted to your left bollock. Nothing is this polished and clean without hiding something.