Why does it take 24 to 48 hours for everyone to see my new site?

When we put your new site live, what we do is update your nameservers to point to the new IP address. When we do that it is instantaneous. However, it takes time for each internet user to be able to see the new site because their computer uses the cache of their Internet Service Provider (ISP) to get the nameservers for your site - and each different ISP updates it's cache after different time periods. Some do it regularly every few hours. Some ISPs only update their domain name cache every few days. As such, if an internet user's ISP has not updated it's cache since your nameservers were changed, that user will go to the wrong IP address.

This is illustrated in the following diagram: