I don't know if there is a given time that Sub is or is not dangerous. I second everything that Lucky sez with the exception of Methadone being harder to kick than Sub as I have done both. Most recently Sub. But then both vary depending on the individual.
Sub is hard on the Liver. If you or someone you know has been on a long time, a simple blood test for Liver Enzyme level could reveal a lot........
Its not necessarily dangerous to be on its just hard to get off the longer you are on it. Suboxone is a very strong long lasting opoid which causes more intense and longer lasting withdrawal then most opiates. (except for methadone) which is the worst. And the length of time that you are on it all depends on the person and what they were using prior to treatment. Are you seeing a doctor to get the suboxone? They will create a schedule for you based on your history of use and body. They will also help you slowly taper off to create as little withdrawal as possible. I do not recommend trying to take it on your own without the help from a doctor. Suboxone is not the drug to mess around with unless you know what your doing.