How Long Will it Take for my Nameservers to be Updated?
In most cases, the Nameservers for your domain name are updated within a couple of minutes. However, due to Nameserver Caching it may take 24 hours or more before you see your domain name resolving through the new nameservers.
Since nameservers do not frequently change, ISP's cache or save the nameserver settings on their servers. Normally, ISP's will only update their records once or twice a day. So depending on your ISP, it may take 24 hours or more before you see your site resolving correctly.
During this period some people will be directed through the new nameservers while others will still be directed through the old nameservers.
Nameserver Updates on .UK Domains
In the case of .uk domain names, the nameserver change must be approved by the registry before the change is completed. This can take the registry anywhere from a couple of minutes to hours depending on how many requests the registry has to process.