Like Stuart said, only 300k? my waxed chains do at least 600k, and if the weather is fine up to 900k.
It's an average and it does fluctuate between seasons, but in the autumn, winter, spring time 300km is pretty much maximum. After that the chain gets too frivolously orange and squeaky.
During the summer I've never seen the need to try how long a chain lasts with a single application.