when i first took oxycodone, without any tolerance to opiates 20mgs orally felt absolutely incredible. yeah, i'd say no more than 30mg, probably better less.
also, responses wildly vary. once, a good friend of mine really wanted to try morphine, and i was quite reluctant as it's just something i don't want to be responsible for if someone ends up hooked, he'd taken quite a few drugs in his life, but never any opiates, after a while i got so annoyed that i ground up a single 10mg pill, snorted about 3/4 or so of it and gave him the rest. there could not have been more than maybe 3mg left and that totally fucked him up, i mean the way i would get from a 15mg iv shot without tolerance. complete loss of coordination, slurred speech, the nods. it wasn't dangerous, i just didn't think that a grown man would feel anything from so little, nevermind such an overwhelming effect. and if i had given him a whole 60mg pill like he asked, half of what i was on at the time, he would have ODd and either woke up in the ICU or died. after that i never, ever again gave hard drugs to someone who wasn't familiar with them.