Lightflow doesn't work correctly on the M8, I tried for hours with this app to get things working properly for a continuous led. While I could do this there was always some caveat that would spoil it.
One of the problems is that notifications don't cancel until you hit the Lightflow notification icon the status bar. I don't want to be doing things twice to read a message and cancel a notification.
I use Recurrent notification manager in Play Store which works correctly and without root for overcoming the 5 min limit.