Check with a dealer for those transmissions. The issues are oil circulation in the gearbox and for some, water to the shaft bearing.
For my Twin Disc the advice was to start the engine every few hours if possible and run for just a short while (minutes). In an emergency I would run for what ever time needed but would not be going that fast.
If your ZF's need to be locked then It might be worthwhile installing a shaft locking system. But avoid if possible - free-wheeling props normally create less drag.