When a domain name is first purchased, or you make changes to the DNS records, it may not be immediately ready for use. The reason for this is propagation. This is a period where ISP nodes across the globe update their cache with new DNS information, which in some cases can take anywhere from 24 to 48 hours for the change to take effect. This includes pretty much any change made to DNS, including modifying your nameservers. You won't specifically define when full propagation is complete because 3 main factors cause varying results.
Those factors are:
Your geographic location
TTL (time-to-live) setting
ISP (Internet Service Provider)
To give you a rough idea of how well the propagation is coming along, you can use a DNS checker to track how far and fast the update has reached. The DNS checker will give you data collected from all locations and confirm if the website has propagated globally. Try these DNS checkers: