mattwnz:timmmay: High pressure hot water cylinders last 10-15 years according to my plumber. Low pressure ones can last much longer.
Our last gas mains one lasted 40 years. The current one in the house is already 20 years old.
All I know is what my plumber told me, 10-15 years for main pressure (ie very high pressure), significantly longer for low pressure ones that have a header tank.