For a 4 digit unlock code, the device would enter them sequentially starting from 0000 and ending at 9999 [1]. Because there is a delay of 6 seconds between each attempt (on iOS 7.xx) it would take just over 16 and a half hours to try all codes.

[1] https://www.fonefunshop.com/ip-box-iphone-password-unlock-to...