Yes it takes 12-48 hours. 72 hours is maximum. It never takes that long.
I am not sure exactly what you mean. Either you change your host;s nameservers or changing your own nameserver's IPs
If you changed the IP yesterdayfor your host's nameservers, it should be up today. I think your registrar messed you up with changing the nameservers or creating nameservers.
If you are changing your own nameservers such as ns1.yoursite.com and ns2.yoursite.com, it may take a bit longer depending on which registrar it is. However it shouldnt take over 7 days. It may take an extra 36-48 hours for the nameservers to glue
It takes 12 to 48 hours on average. I think they say 7-10 days to set expectations for propagation. I have registered multiple nameservers through registerfly and most of them are available in 18 to 24 hours.