How long should a chain last?

I fitted a new chain and sprocket kit last summer. Since then I've done around 8000 miles and stuck with a general rule of lubing the chain every 500 miles or so. It's given me no trouble at all untill suddenly last week it's gone slack? Do they stretch suddenly or wear over time to the point where you notice it? I check it regularly and it's been fine but suddenly it's got twice as much play as it used to have?

Generally you shouldn't notice a sudden difference mate? Not sure why this would happen. I would adjust it then see if it get any slacker after a decent ride out, I've had chains last for 14k miles before, as long as you maintain it, it should last!

i like to ride my bike in the winter get it covered in salt never wash it and have to change my chain every 6 months