As for 4-aco-dmt, the PIHKaL range is 10-25mg, iirc. Those tend to be a bit on the low side. I've heard reports that 20-25mg is where things get very intense, and anything beyond 30 or so is probably excessive by most people's standards.
Back to 2c-i... Yeah, I've never come close to ego death, merging of self and other or any of what I would call the ontologically significant psychedelic effects - body high, altered head space, weirdly subtle 2c-i visuals (almost reminds me of the vivid mental imagery that tends to come up on cannabis, except the distinction between mental imagery and actual vision is less clear and stable while tripping).
My usual chronology is something like this (assuming empty stomach, comeup is delayed otherwise) -
t+0.5h : first alert
t+1h : definitely on the way up by here
t+2h : by now I'm at or at least close to peak/plateau levels.
t+7h : this is around the end of the plateau - no specific effects seem gone by this point, but it begins to feel like a transition from tripping mental state to post-trip contemplation mental state and the intensity begins to drop off
t+10h : by this point the effects are largely gone, I'd be willing to drive for something other than an absolute emergency (although I do my best to avoid driving until after I've had a good night's sleep whenever I use any powerful drug), but if I smoke a lot of pot I can get a resurgence of visuals. There's still a lingering body high, mostly consisting of stimulation with a bit of that unique 2c-i feel to it, which fades slowly over the next couple hours.
In general, I tend not to notice nearly as abrupt an end to any intoxicated state as many others seem to. I notice this the most with pot - I think my sensations of 'high' and 'burnt out' blend together just enough that it's hard for me to distinguish between what others would consider lingering after effects versus just a slight burnt out feeling after it's worn off. With psychedelics, I usually experience a gradual decline in intensity, but I don't feel 'sober' until the next day.
As for 2c-i tolerance... There's probably some, at least subjectively, but I haven't noticed anything dramatic. I have tripped two nights in a row on the same amount and, while the second night seemed a bit less intense, it certainly didn't feel like a significantly smaller dose or anything - more like it skipped the 'wow!' moment of the first hour or so of the peak. I've tripped multiple times over the course of a week and didn't notice any tolerance more dramatic than that.
carpaljets: I'd love to hear more about the OEVs your group experienced. Did any of the experienced trippers comment on its similarities/differences to other psychs they've tried? Any encounter with the 'subtlety effect' I've stumbled over describing here a couple times?
edit: currently t+12h on 30mg, ate a light meal shortly before dosing, still strong ++ / borderline +++. As with other 5HT2a psychedelics, cannabis seems to be particularly useful for extending the experience as well as intensifying and altering the trip.