Name servers may be updated from 1 minute to 10 days, depending on the zone. Immediately after Your order, You may not see a change in NS servers.
Such changes are instantaneous, but domain registries change them over time. In addition, Your Internet provider has a special DNS cache. That’s why NS updates can take from 1 minute to 72 hours, depending on Your Internet provider.
You can check the status of the NS change on the website www.whatsmydns.net/