There are a couple things that I want to throw in here. (I've been violently ill, still am..) I would never totally recommend eyballing it. If you get your tolerance up (which will happen with fentanyl, very quickly) and get to know how big your bump is and you are snorting or smoking most likely youll be okay. I would never suggest doing this because EVERYONE reacts differently to opiates. Just because one day you are ok with X dose, doesn't mean you will ok with that same dose today. Another thing i want to point out is that Fentanyl has almost no legs. It hits hard and fast, but can go away fairly quickly as well. I am ok with fentanyl, but its not my favorite because of this. I have to re-dose multiple times a day. I would say the average time it lasts in about 30 minutes to 45 minutes. Thats not a long time, But you shouldn't go thru withdrawals for a few hours if your physically used to it. I can't tell you how many people that I know personally ether loves fent or hates it for the above reason. Most people shoot it because you can take such a small dose and will have many doses. Snorting is not even half as potent as shooting and even less smoking. I'm not trying to diminish fents dangers at all. I just want to warn people that they may have to redose multiple times to stay as high as they like. Also, fent raises your tolerance like crazy, so don't be surprised if you have to up your dose after only a day or two of continual dosing. 80 percent of people love fent, The other 20 percent just like it (Im in the 20 percentile). IF you understand how fent works most everyone will at least like it. I've had people say that it got them high but not as high as everyone is posting. You have to remember that everyone is different, and can react differently to every opioid. I know many people that will get high as hell off an 20mg of hydrocodone yet have to take a ton of oxy and prefer hydro. Some people are just the opposite. That's why there are so many opiates out there. If one fit everyones needs, pharmaceutical companies or doctors would stick to one pain medication. Still, to be safe, its always best to measure it out with a milligram scale, the ones that go 3 decimal places. They are very cheap over at amazon, it only takes about 3 to 5 days to have one mailed to your home.. Its a wise and good investment.