I used to use Oxycodone to help me fall asleep. I did that every night for about a year, I'm still kickin'.
Really, unless you take more than you can handle (which is why nobody can give an exact number), you're fine.
IME and from every piece of literature I've read, Opiates are actually quite safe. They're really only dangerous when you overdo it or mix them with other CNS depressants, and even then, a lot of people get away with it because of tolerance and experience. I've also read, specifically, that your body builds tolerance to the respiratory depression associated with Opiates. I've read it's much easier for an opiate naive person to OD than a dependent user. You get all this hyperbole about the dangers of drugs because people can and do use foolishly and with little regard for safety, so in turn you get the "knee jerk" stigma reaction from DARE types that translates into, "ZOMG IF YOU USE DRUGS YOU DIE FOO'."
Regardless, I'd rather be an Oxy addict than a cocaine addict any day, speaking strictly from a "concerned about my health" standpoint.
You have to remember, millions of people are chronically prescribed Opiates to live their lives and free themselves from severe pain (myself included), if they were that dangerous they wouldn't be the go-to pain meds. If anything, the biggest danger with Opiates is addiction which can severely destroy your willpower and if you're not careful, Opiates become your god, king, and savior.
Sounds like DARE bullshit to me, OP. Just don't use more than you know your body can handle specifically if you're using before bed. Save dose experimentation for your waking hours, and as always, be careful!