​How long do Ethereum transactions take

One day I decided to withdraw my Etherum coins on usb-wallet. I launched myetherwallet and ordered the transferring money into another wallet. I leaved the computer for 15 minutes. After my return transaction is not completed. What is wrong? I think that I had transfered Ethereum coins more quickly than it had before. What is your experience? How long does it take to transfer Ethereum from one wallet into an another wallet or maybe into some pay system. Tell me please what anybody know how long does an Ethereum transaction take commonly?