on HD2 which is stuck with the older FW 5.20, the max value is poweroff=4, .i.e 60min. I remember having tried to play with it at no avail, and I assumed that the times corresponding to the enum are hardcoded somewhere, at least because I have not found any other possible settings file in clear (
I looked also in /ebrmain and /mnt/secure at that time). But reading other posts, AFAIR, devices with FW 6.X have more options, and IIRC HD3 got the update to 6.X. Are you on the latest firmware for your device?