Smoking time is a range, not an exact figure. A small cigar such as a tres petit corona can be finished in about 20 minutes, and a petit corona in about half an hour. A robusto is commonly said to take about 45 minutes to an hour. Large sizes such as the Churchill and the double corona can take an hour and a half to two hours or more.
Length matters, and so does ring gauge, because a thicker cigar holds more tobacco per inch. After size, construction and storage matter. A tight draw slows the cigar down, and a loose bunch or a dry cigar burns fast and hot. Your pace matters most. Puffing every few seconds shortens the cigar and heats the smoke; the common advice is to take a relaxed puff about once a minute.
Choose a size that fits the time you have. Putting a cigar down before it is finished is fine, but a cigar smoked in a hurry rarely tastes its best.