There's been some mention here of group categories of pipes, 3,4,5, and so forth. I was unaware of these groups. I assume they refer to bowl sizes. How do you tell what group size your pipe is? I have mostly Peterson billiards, about four of the 107s: what size would these be considered? BTW, my smoking time, if there are not any interruptions, is about 15-20 minutes, 35 or longer if I'm smoking (rarely) my giant Nording. A Peterson 107 lasts me about 15-20 minutes.