We ask several independent resolvers the same question at once and compare their answers. If they disagree, some are still handing out your old record.
It isn't your record travelling across the world. The moment you save a change, your authoritative nameservers have it. What takes time is every other resolver letting go of the copy it already had.
Each resolver keeps your old record for as long as that record's TTLsaid it could. Until that runs out it keeps answering with the old value — correctly, by the rules. So the honest answer to “how long?” is: about as long as the TTL on the record you replaced.
Most propagation checkers plot resolvers on a map. It looks great, and it's misleading: the big public resolvers are anycast, so 8.8.8.8 answers from whichever of Google's locations is nearest to whoever asked. Checking it “from Tokyo” reaches a different machine than a person in Tokyo would.
What genuinely differs between the resolvers we ask is their cache. So that's what we report: which operators agree, and which are still holding the old answer.
Some domains hand out different addresses on purpose. Big services route you to whichever data centre is nearest, so the answer genuinely depends on who's asking — ask about google.com and you'll get a handful of different addresses back, today and every day.
There's no way for us to tell that apart from a change still working its way through, so we don't pretend to. If the resolvers all agree, you're definitely done. If they don't, and you've just made a change, it's almost certainly caches catching up. If you haven't changed anything, it's probably deliberate and it will never “finish”.
MX cutover isn't visible in an A lookup.Once it's settled, a DNS monitor will tell you if it ever changes again without you.
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