On the minecraft.net website when you enter your profile and will change your minecraft nickname, before you change, a note appears saying: "It is only possible to change the name once every 3 days. Choose your identity well!" 
https://i.imgur.com/en53tSO.png

The right thing would be 30 days. And right after you change your nickname, it goes back to normal
https://i.imgur.com/1RTNLIy.png