How log does it take for customer name servers to be updated in the registry once you've added them?

24-48h hours...

as long as you mean e.g. ns1.YOUR-DOMAIN.xxx

changing a nameserver to another one usual takes a few minutes with com/net/org

Depends on location. FOR US and West European countries, the DNS changes are almost immediate, but for Asian countries, it might take up to 4hrs or maybe more to propagate.

I change DNS and can access the new location immediately from my US and Germany based server.

Looks like it only took about 8 hours locally, 24 hours internationally.

Yeah if they register it with their domain through the registrar its different per registrar. Some update immediately to the root servers and some do it on a schedule once or twice a day.