What is the general lifespan of a well maintained Chain ?
I have had my current bike nearly two years and ridden 13,000 km
If you've done 13,000km on typical UK roads and weather then you now need a new chain and cassette.
If you leave it too long to replace a chain then you will have to replace the cassette too.
I recommend using a Park Tool CC2 chain wear checker
Chain life discussed plenty here:
You may need new chainrings and jockey wheels to go with that new chain and cassette too. A chain might last 1500 - 2000 km depending on how you ride and the weather you go out in. If you buy a chain wear indicator and check your chain regularly you'll be able to change your chain before you do any damage to the rest of your equipment by riding with a worn chain. Chains are a consumable item and they need changing. The more expensive ones won't necessarily last longer. I find that Shimano 105 and Campagnolo Veloce chains offer the best combination of price and durability. KMC X- L chains are convenient as they can be easily removed when cleaning your bike although the mechanic at my LBS doesn't rate them at all. I've found that SRAM chains seem to wear out quickly but others may have had better experience with them.