How do you measure very long half-lives?

Answer 1

You cannot measure it, you suppose that the decay law works all the time. Then using the half-life definition (the time which you have to wait to get half of all nucleis decayed) just put all your data in the formula and calculate what you need. The decay all was checked for not too long periods of time. And people suppose that it works for long times also. It's no reason based on multiple experiments to think that something is wrong with this law.

Answer 2

To measure very long half-lives, you can use extremely sensitive detection methods, such as mass spectrometry or gamma spectroscopy. These techniques allow for the detection of low levels of radioactivity over extended periods of time, enabling the measurement of very long half-lives in radioactive decay.