Machine Drive Chain

My guess is that your asking about roller chain and preventive maintenance.

US Tsubaki had at one point very good information, I'm sure they still do. Remember quality of the roller chain does have a big affect.

http://www.ustsubaki.com/technical-resources.html

Chain life is usaully determined by the amount of stretch. With the chain laid out on a flat surface, compress the links, then pull them apart and measure the stretch.

The source 911 gave you is very good.

Chains are checked to see how much they have 'stretched' with wear. A quick check is to see how slack the chain is between sprocket tips. At the middle of the sprocket try to lift the chain away from the sprocket. If it lifts away, it is worn.

The chain dealer can give you the specs on allowable stretch, and how to measure it.

As far as time, it is a factor of use, lubrication, and cleanliness. A timing chain in a motor is going to last a lot longer than an exposed drive chain on a motorcycle.