DNS Updates

Hi Everyone, I was wondering if anybody knew when most DNS servers update their records with regard to changes in DNS hosts for a domain. I've made changes to my domain, and it looks like only a handful of DNS servers (earthlink being one of them that I could find) have actually made the changes. I've heard that the turnaround time is normally 24-48 hours for the changes to take effect, yet it seems that it takes FOREVER for anything to ever happen when I have NSI update the records for the site. Is there a reference on the net somewhere containing when some of the major DNS servers update their records, or does anybody have any idea why it always takes so long? Many Thanks, Ben