It will only take time. CRTs just degrade over time. You shouldn't see this for a really long time though. Like 4+ years or more. But if you see it rather quick something is going wrong and needs to be addressed by the manufacturer.
As is the case with all newer model of monitor (crt or LCD) they have a built in off time length, meaning that after x hours of unuse, the monitor switches off on it's own. If you mean to be on the PC for the entire time, then you may just have a computer problem (addiction?). But I have never seen a monitor overheat in all of my years, except in the case where a bunch of sawdust fell into the casement and the monitor caught fire... That was the only time...
Hope that helps.