Good question. I do not have it at hand, but I try to describe from my memory as well as possible....
1) Invoke a menu by touching "button", which is in the upper left screen corner (I think 2nd from the left). I do not remember the name, but it is sort of "strange" - non-intuitive.
2) The last or last but one item in menu invokes dialog for setting power-off and sleep times.
I do not have the latest OTFM version installed. It may be little different in the latest one... Simply search the OTFM menus and you will find, where the time setting is.
That is all. :-)
And it is great! My setting is 15 min sleep and no power-off, which is not possible to set
from standard Onyx interface.
It is 3rd menu item from left - "Trinkets" in older versions or "Device" in 0.6 version