The default TTL is 3600 seconds. Donating users can change the TTL to as low as 120 seconds. (TTL is "Time To Live", and it controls how many seconds an ISP's caching nameserver may cache results that it received from one of the ns{1,2,3,4}.everydns.net authoritative nameservers.)
Changes are sent to the secondary DNS servers (ns2, ns3, ns4) very shortly after changes are saved on the website, but it may take longer for existing records on other DNS systems (say, at an ISP) to reflect these changes. Wait an hour or so and you should see your changes take effect.