How often is it that someone who gets "legaly" high will move onto illegal drugs?

This is another question about my friend who has some problems. The first time i heard about her messing around with drugs was last year when she tried to snort tylenol. Needless to say she didn’t get high. But since then she has moved on to huffing dust-off and as I’ve recently found out she has been taking excess amounts of OTC sleeping pills then trying to stay awake. I want to know how often it is that someone who starts out like this, will end up moving on to drugs like pot, cocaine or something.