How fast should a site start working after connecting cloudflare?

Question

Domain Name System

Fighting off DDoS in the heat of the moment, I decided to try CloudFlare. On the CF website, I added my domain, added the missing DNS records, at the registrar, instead of his ns servers, I registered the two that CloudFlare gave me.
Some time later, I received an email from CloudFlare that my domain was successfully added. On the CF website, a green checkmark appeared next to the domain, saying everything is ok. Immediately, the site crashes and becomes unavailable... (hereinafter, mydomain.ru means my real domain).
Chrome: ERR_NAME_NOT_RESOLVED
ping: unknown host mydomain.ru
nslookup mydomain.ru 8.8.8.8: no address
nslookup mydomain.ru ETTA.NS.CLOUDFLARE.COM: returned 2 CloudFlare IPs
I go to https://who.is and check the domain - new ones CF ns-servers are visible.
dig @ 8.8.8.8 mydomain.ru: no ANSWER
dig @ETTA.NS.CLOUDFLARE.COM mydomain.ru: returned 2 CloudFlare IPs
http://dnscheck.pingdom.com: everything is bad
I could not stand different DNSs and gave up, returning its ns-servers from the registrar. The site and domain instantly earned in the previous mode.
Actually it is not clear - did I do something wrong? Is it generally normal that for so long after changing ns-servers to CF-ovskie the domain does not resolve to the CF proxy IP and nothing works at all?

Answer 1

Still, I decided to wait at least 12 hours to check whether it is realistic to switch to CF. Around midnight, when most of the users left the site, switched dns servers from registrar's servers to CF's dns servers. As with previous attempts, the site almost immediately went offline, the DNS zone broke and was empty. I put my domain in the Selectel monitoring for DNS check, and by the morning I saw how responses with CF addresses began to appear from Frankfurt and Amsterdam, hooray! From other places, absolutely emptiness returned - no address. Accordingly, the further, the better it got.
As far as I understand, the problem is in my registrar, and specifically in the fact that after setting up other dns servers (other than the registrar's servers), the zone from the registrar's dns servers is instantly deleted and the domain turns out to be in complete, sorry, asshole. Guaranteed downtime is provided. Keep the registrar in your dns zone for some time, at least the same 72 hours, everything would smoothly move to CF.
Such is the story.

Answer 2

After adding the domain, go to the DNS zone management and create the necessary A-records. Perhaps CF didn't do this automatically, as it should have. It usually scans the old zone and parses data from there.