There is no one single answer. Blade lengths may vary from no limit in some states to a 2.5" limit in places like Chicago, Boston, Cleveland, all federal facilities. I would argue that more important than simple blade length, is using common sense. Do not flash a knife when not necessary. Keep your car in shape so your are not pulled over and a knife is discovered during a search, and don't go to bad places or do bad things. I know that this is common sense, but it bears repeating. In my opinion, discrete carry of any knife under 3.5" will probably notcause problems, assuming that you are okay otherwise.