Very old question but still comes up #1 on Google for "how long does it take to smoke a pipe" so...
To answer this question more specifically and to the point, it takes about 8-10 minutes to actually smoke an average pipe, with average cut and size.
What is done in between "puffs" (the actual act of smoking a pipe) really depends on your preference. I did not want new users to think you can puff on a pipe in a constant manner (one puff every 30 seconds) for 45 minutes - that would take a pipe the size of a coke can.
Everyone above (110% correctly stated) takes X minutes for this relaxing "ritual". Been smoking about 30 years and sometimes I will light a pipe and smoke for 5 minutes. Sometimes I will put it down and go slowly for 30 minutes. I do not think the pauses in between puffs count for much. Similar to saying how long does it take to eat a sandwich and someone says "1 hour" because they have 50 minutes of resting time in between bites. YES, their process of finishing a sandwich was an hour, but it took them 5 minutes to actually eat the sandwich. I've seen others swallow one down in 3 : )
To the point, if you're new to this, take your time. It is not supposed to be a rush to the end but there is nothing wrong with taking a few hits and putting it down.