Yeah I suppose a person's situation plays a huge factor in what they are refering to by "how long"
For me, I needed to 100's of mg's per dose just to feel ok. I was at the point where NO dose, no matter how big would give me euphoria, doses either kept me well or put me to sleep/into a nod---but not that euphoric nod you get in the begining, just a sort of my eyes won't stay open nod.
When I started with opaites, before being physically dependant, an oral dose oof oxy would have me "high" in a pleasant, buzzing, social, very recreational way for about 3 hours, with the afterglow lasting until I fell asleep.
When I first got physically dependant but still enjoyed the highs, a dose would have me high for about 2 hours with some nodding the last 2-4 hours.
When I was very addicted, many years into it, 2 relapses into it, the high would last about an hour. And. When I was dependant on very large doses, I would only feel "high"-ish, for 20-30 minutes, basically the time when I could feel the drugs kicking in.
But this wasn't the same high as before. Its the same with methadone for me now, I wouldn't call it a high, but for about 30 minutes when the methadone is coming on, I can "feel" the drug working. And even though I'm just going from methadone at low levels to high levels, just feeling a change is kinda like a high. (I'm on 140mg per day for about 1 year now)
So I guess it really depends on ones own unique situation, but oxy does definately have a more energetic buzz than dope at leeast in the begining of ones use, so it does feel like the "high" fades quicker than the high from dope. But this is b/c the dope high is very sedating and it is tough to tell where the high fades and the afterglow sets in with dope. Whereas with oxy, that line is more pronounced.