DNS propagation is the time it takes for a DNS change — like updating an A record or switching nameservers — to be visible everywhere on the internet. There's no single fixed number: propagation typically takes anywhere from a few minutes to 48 hours, and it's governed almost entirely by one setting: TTL.
TTL (Time To Live) is a value set on every DNS record, measured in seconds, that tells resolvers how long they're allowed to cache that record before checking again. If your A record has a TTL of 3600 (1 hour), a resolver that already cached the old value won't ask again until that hour is up — so anyone relying on that resolver keeps seeing the old value until it expires, even though the DNS record itself changed instantly.
Every resolver in the world — your ISP's, Google's 8.8.8.8, Cloudflare's 1.1.1.1, corporate DNS servers — caches independently and on its own schedule. That's why you might see the new value from one resolver immediately, while a colleague on a different network still sees the old one for hours. Nameserver changes tend to take longer to fully propagate than a simple record update, because nameserver (NS) records themselves often carry longer TTLs, sometimes 24–48 hours, at the registry level.
If you're planning a migration (new server, new host, new mail provider), lower the TTL on the relevant records a day or two in advance — down to something like 300 seconds (5 minutes). Once the low TTL has had time to propagate and old cached values expire, make your actual change; it'll then be visible almost everywhere within minutes instead of hours. Remember to raise the TTL back to a normal value afterward for efficiency.
Our DNS Propagation Checker queries 20+ resolvers across different regions simultaneously, so you can see in real time exactly which locations have picked up your change and which are still serving cached results.