well the way my ol man told me was that when it starts affecting you financially then you have a problem. costing you monies for any related offenses, like DUI, fighting at bars, etc. this also includes missing work, costing you monies.
Id say it becomes a problem when it starts affecting your ADLs (Activities of Daily Life) aka you aren't functioning at the same level you were before. Also, when you feel like you HAVE to use in order to get through the day (psychologial depenedence). And also if you experience physical withdrawal symptoms when you stop using (physical dependence).