I added two NS records to my DNS record four days ago.  I did this to use a service provided by the name servers that I added.  In order to get them to enable their services, they need to verify that they are name server for my domain by doing a whois on my domain.    It has been four days and whois still does not show the two new NS records.  The whois server that gets checked is whois.rnids.rs.Is there something special I need to do to update whois?  I've verified that the new NS records have propagated to many DNS servers by nslookup -q=NS <my domain>.  Indeed, the two original and two new NS records are returned.  
How long does it take a whois server to update?
whois
It varies from company to company. I have spent quite some time with GoDaddy, NameCheap and Whois.net. Once I updated nameserver records for a domain registered with GoDaddy, it barely took 2-3 hours. On contrary, NameCheap took almost 2 days to update the records for another domain I was configuring.I think the best way is to ask the domain registrar before changing the records. This way, they would take any necessary manual steps involved before the bots kick in.