How often do I change timing chain on 93 Nissan sentra?

On the 1.6 litre Sentra engines you should not have to change a timing chain until well over 250,000 miles if ever. This engine has an automatic chain tensioner that keeps the chain tight so that it will not jump teeth when stretched due to wear. I have heard of early 90s engines developing gear tooth cupping that prevents the engine from retaining proper ignition timing. This causes the engine to run a little rough. I have never SEEN this though.