I found a good answer:
How Often to Replace Chains
The rule of thumb is you should replace your chain BEFORE a foot of chain has stretched to the point where it is a foot and one eight of an inch.
Checking this is Simple:
- Shift to the big-big gear combination. (Gives a nice long place to measure).
- Lay your bike on its left side.
- Lay a one foot ruler or tape on the top run of chain such that the Zero mark is dead center in one of the chain pins. Hold it there very steady.
- Check the other end of the ruler. On a new chain it will be dead center on another chain pin.
- If the chain pin center is less than 1/16th beyond the end of the ruler it's OK.
- Between a 16th and an 8th, you should think about changing the chain at the earliest convenience.
- If the chain pin center is 1/8th of an inch beyond the end of the ruler it is past time to change your chain.
You may get anywhere from 2000 miles to 20,000 miles on a bike chain before it needs replacement. It all depends on how much grit and dirt accumulate on the chain and your style of pedaling.
http://www.bicyclinglife.com/HowTo/ChangeAChain.htm
Tom
goodtri.org
