How long will it take to switch nameservers?

Max. time for a change to your Authoritative Nameserver to apply worldwide:
48 hours.

If you've made a change to your nameservers that doesn't seem to have taken effect, this is probably why. 

If you change your Authoritative Nameservers in Porkbun’s Domain Management console, this change can take 48 hours to fully propagate across the Internet. Porkbun immediately submits any Authoritative Nameserver changes you make to the appropriate registry, but after that it’s pretty much out of our hands. To check the Authoritative Nameserver propagation, you can use a DNS propagation checker such as https://www.whatsmydns.net/ and select “NS” as the record type.