On a related note: how do you know when you're warmed up?
I think there is no straightforward yet helpful explanation. It's when you feel (or your biometric devices tell you) that you are ready to do what you want to do.
If you are starting a yoga session then you might do a bit of limbering up.
If you are starting out on a jog or a 'leisure' cycle ride then you might gradually increase your pace until you feel that you would be unable to complete a spoken sentence without breaking for breath.
And if you are on a gym cycle trainer, connected up to heart rate monitor, power meter, cadence meter and the rest you might specify a target combination of percentage maximum heart rate (eg 70%) with target power and cadence.
If you focus on assessing your warm up then with experience, you will confirm for yourself that what you use gets the results (performance or fitness) that you want.