Originally Posted by
HendersonD
How long does a chain normally last? Is it necessary to carry an extra chain for a 3 month tour?
No definitive answer. I'd say that one chain might do, but the conventional wisdom is that an elongated chain will damage your entire drivetrain (i.e. will necessitate a new cassette and perhaps a new set of rings as well.) Claimed longevity vary from, I'd say, 1000-5000 miles. More common would be 1500 miles.
On longer tours I carry a spare and rotate them.
Better answer would be to check elongation from time to time with a decent chain checker, and get a new chain whenever your current chain is worn. (i.e. elongation > 0.75%)