It depends on your pipe and how fast you puff. I am a medium puffer. Here are some estimates, based my own experience.
MM Minis/Tom Sawyer/Shortstop/Morgan/Nosewarmer-a great 10-minute smoke, for a break or a quickie smoke.
MM Eaton/Pony Express/Dwarf/Huck Finn-around 15, maybe 20 minutes. Again, great for when your smoking time is limited.
MM Legend/Washington/Patriot/Shire/Elf- 25-35 minutes. A nice average-length smoke.
MM Country Gentleman/Diplomat/Mark Twain-30 to 40 minutes
MM General/Freehand-45 minutes to 1 hour
MacArthurs-1-1/2 hours+. These things hold around 1/2 oz of tobacco! Load it in the morning and smoke it all day....
I have one pipe larger than the MacArthurs. It is a Mr. Brog Lumberjack...basically a hollowed-out tree stump with a tenon and and a stem added. I have smoked it as long as 2-1/2 hours, and it holds the best part of a 1.75 oz. tin of tobacco. I smoke this (and the MacArthurs) usually when I am out catfishing from the bank. I don't think there is a larger pipe that would actually be smoke-able. Anything larger would probably require a Bonfire Permit to light. The bowl on my Lumberjack is almost big enough to cook over. If they made a grill for it, you could probably cook a hamburger on it while you are smoking....