There's [a lot of information here][1] about why chains wear, how long they might last, how to tell when they need replacing, and what happens if you don't when they need it.

* 1000 km (cross-country, or all-weather abuse)
* 3-5000 km (well-maintained derailer chains)
* more than 6,000 km for perfectly groomed high-quality chains, single-gear, or hub-gear chains (preferably with a full cover chain guard)

So if you're doing 800 km/month, then you ought to replace chains maybe monthly if you don't clean and lube them every week, otherwise 2-3 times/year.


  [1]: http://en.wikipedia.org/wiki/Bicycle_chain#Wear