Hello
This is dependant on your time to live on your records. Nearly all ISP's resolver nameservers do 'cache' nameserver records, so that they do not have to communicate with the domain's nameservers everytime they need to resolve a record. This way is efficient, and once the TTL time has passed, and another person requests the record from that nameserver, it grabs it from the parent nameserver of the domain, and stores it for the amount of time that the TTL is.
Hope it helps
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