On my 2012 iMac (Sierra), you hear the boot sound and press the option key => a few seconds later it gives you the boot disc option.
Using the iMac 2017 (High Sierra - keyboard wired), there is no boot sound anymore to trigger on. So I press and hold the option key 1 second after power on.
In case a restart of the system (Win10 or MacOS - wireless keyboard) is triggered, press and hold the option key (just after reboot - when the screen goes really black) does not always bring me the boot disc option. I frequently boots the MacOS...