The ATIS is usually only updated hourly, unless something happens that needs to be broadcasted before the hour is up.
Here in austin, they'll often make a new atis broadcast for "birds on and in vicinity of airport". Other reasons would be a wind shift (especially one major enough to change the active runways).
If the visibility is suddenly reduced due to precipitation, or low cielings suddenly move in, you'll also get a quick change on the ATIS.
Sometimes, like tonight, you'll listen to the ATIS, and right after you switch frequencies to go on approach ATC will put a new atis on. So you'll say "approach, cessna NXXXX at XXXX over XXXX with uniform", and ATC might say "cessna NXXXX Squawk XXXX, fly heading XXX, information victor is now current" so then you'd listen to the ATIS again to get victor.