How long does it take for a MX record to be working

The accurate answer is that DNS updates are instant. Once the records are on the server, anyone who queries your domain will see accurate information if they haven’t cached another reply before.

What people refer to as “propagation” is actually just cached information doing the job its meant to do: caching. Once those caches are expired, they are dropped and the next time your entry is resolved, you will see the correct result. This is why when you test a domain you’ve used recently, it seems to show old information even after an update.

Now, there are other factors that take part in this. For instance, DO batches updates to their name servers, so it may take a few minutes before they actually start responding with your latest zone. Also, your name server records on the TLD servers (.com, .org, .net, etc) generally have a higher TTL and thus their cache gets cleared later than you’d expect.

This is pretty much all I can say, since you didn’t give us anything we can test with like the domain name.