I have a handful of instances where I remember having a block so long I really got nervous and the people or person I was talking to sort of lost their patience and I felt extra frustration (people who knew full well that I stutter). I mean, I even remember the word that was the trouble and in some cases it's been over 20 years. That is to say, normally my stuttering isn't nowhere near as severe — and, having said that, and having met a decent variety of other stutterers, I'm convinced that some of them have those kind of experiences at least weekly if not daily. That's one way I'd define severe stuttering.