I find that a worn chain ring gets sharper and sharper. When, you are likely to get cut putting a chain on it, it is probably time to replace it. A chain ring usually lasts over 20,000 miles. A way to get more wear out of your rear cluster and chain ring is to change your chain before it gets overly worn. You can buy a little gauge for about $10 at your LBS that makes measuring chain wear on the chain very easy.