Nameservers propagation can take between 24 to 48 hours approximately across the globe. If you have just purchased your domain and set up the nameserver, your website will remain down until the DNS propagation complete.
What is DNS propagation? How can I check DNS propagation?
DNS propagation is the time to update the DNS changes on the internet. It could be the DNS record like A, MX, TXT, or Nameserver records. During the propagation period, DNS records for a domain update from the local DNS resolver to the chain of global DNS servers. Please refer to How the DNS works?
Please refer to DNS Propagation Checker to check your domain propagation. You can also select the individual record type.
Why does DNS propagation take time?
You can consider the below factors that can affect DNS propagation.
(1) Domain Registrar :
Once you modify the nameservers at the domain registrar (From where you have purchased your domain), it must reflect TLD root servers. Suppose you have a domain name with .com TLD, then the nameserver will publish from local DNS resolvers to higher root servers. Most of the registrar instantly reflect the nameserver, but some might take a few hours to one or two days.
(2) TTL (Time to Leave) Settings :
DNS Servers allow setting TTL. It is time the remote server can cache the domain's DNS records. Suppose you have set an A record with the TTL of one hour. It will store the DNS cache locally for an hour. After one hour, it will retrieve the updated record from the domain DNS record. You should set the shorter TTL to speed up the propagation. Higher TTL time can cause a propagation delay.
(3) ISP (Internet Service Provider):
ISP stores the DNS record locally instead of retrieving fresh DNS entries from DNS servers. It allows them to speed up web browsing. However, it will slow down the DNS propagation. Many ISPs ignore the Domain TTL setting and update the DNS cache regularly, like two or three days.