Many of the changes you make have immediate effect. The ones that don't tend to involve DNS or domain registration changes.
Most DNS changes, such as associating a name like www.example.com with a site hosted here), take effect within a minute or two. However, there is a catch. If you recently accessed the name before you set it up, your browser and/or local ISP may "remember" the old information. (Either that it pointed somewhere else or that it didn't work.) Exactly how long that will last depends on what the previous setup was; if it was moving things around on our system it should be visible within a couple of hours. If you changed a domain from somewhere else, it may take a day or more depending on what the other provider had set.
How long it takes to see DNS changes is controlled by the time-to-live (TTL) value for the relevant DNS records. (If a record doesn't exist, the domain contains a setting called Minimum TTL which will be used to tell people how long they can assume that record will continue not to exist.) When you look up a domain name, your local name server (usually run by your ISP) will look at the TTL value and will keep giving you the same answer for that long, even if there are changes in the meantime. (Advanced members with the right tools -- such as "dig" -- can query their name servers to see how much time is left before a given record is rechecked, but that's beyond the scope of this FAQ.)
The root DNS servers operated by the domain registries store your name server records with TTLs of two days. That means if you change your domain's name servers, that change can take up to two days to be fully visible throughout the Internet. (The same applies if your domain expires and gets the "parking" name servers, so don't let that happen!)
