first of all you really shouldn't IV suboxone, if you couldn't tell by the rapper and by what your doctor told you, it has 2mg of naloxone in it so that you don't IV it. of course buprenorphine has a much higher affinity than naloxone, but the naloxone will certainly bind to some of your receptors, causing antagonism WHILE the buprenorphine agonizes other receptors. it's a pretty muddy high for that reason. anyway, regardless of the route of administration, suboxone is extremely lipophilic and is not easily displaced from receptors. for those reasons, no matter what way you do it, it has a half life of at least 12 hours. however the time it takes for suboxone to move from your tongue to your brain is highly variable. if you press it down against your veins it will hit you more quickly. if you coat the bottom of your tongue in oil first (and theoretically alcohol would work too) it will also hit you more quickly. i have used oil many times, not sure about alcohol. this is obviously because lipids cross membranes rapidly and suboxone is soluble in lipids. so the oil carries it directly into your veins and directly through your blood brain barrier. injecting suboxone will still hit you much faster than this method and since it hits you just about all at once, the half life will be slightly shorter. but your friend is wrong about the 6-7 hour thing... suboxone, regardless of how you take it, will mostly stay bound to your receptors for MUCH longer than 6 hours. we're talking like 36 hours.... the idea that it could unbind in quantities substantial enough to produce withdrawal in 6 hours is completely wrong. clinically, and in my experience, suboxone takes at least 48 hours before withdrawal symptoms even begin to occur. most users can go a full day without suboxone without experiencing any dope sickness. 2 days for me. the 3rd day is when you start to feel sick, and the 5th to the 7th days are the worst. ive shot up subutex many times and it affected me considerably for at least 12 hours every time. the only difference would be the naloxone in suboxone, but i think if that were the reason your friend got sick, he would have gotten sick immediately. naloxone's half life is like an hour, so it should be totally metabolized within a couple hours. your friend is probably imagining it. it is possible that your friend is noticing a decrease in the plasma levels, since shooting up buprenorphine produces an immediate rush, and it kinda plateaus for a bit and then slowly drops over the course of a day. you'll usually wake up the next day feeling normal (not high, but not sick), but in fact there is still plenty of buprenorphine bound to your receptors and you just don't notice it.
edit: im pretty confident in my answer too, being a former user of opiates and suboxone myself, and being a biochemistry/molecular biology major with pharmacology minor. im on my way to being a "drug scientist" so i'm pretty confident with what i typed.