So far every customer I've been to has asked for the time out period to be set to 5 minutes.

If the operator is to lazy to even press a button to switch the truck of & instead having it time out when leaving it parked up during break times / end of shift then that's an issue for there manager to deal with.

i'm not sure on legislation on this feature but in most cases i've seen when it comes to the truck timing out after the operator leaves the seat, it generally is set at a default time of around 30 seconds.
It is usually adjustable and can be shortened or lengthened depending on the desire of the owner. I'm pretty sure as part of the OPS (operator presence system) required to be implemented on machines now it just has to be present on the machine and operational.
There is alot of info to sift through in the OSHA and ANSI standard data and finding that particular reference is cumbersome. Maybe someone else already knows of this and can shed some better light on it. ;o)