I've bought a couple of mountain bikes from Amazon for about $250, basically chinese bikes. They seem fine enough, I buy them basically for work and do about 20-30 miles a week on them. I am a bit of a high gear pusher.

After about 18 months they start getting loose or creaky / noisy at the crank shaft / axle area every time. Is this just a case of cheap materials grinding down and life expectancy or do all bikes start to fail there after that amount of time and it's just a matter of replacing the axle/crank stuff?

Keep in mind this is for work, I'm not a biker. I don't know how to fix bikes.