The japanese hinoki is a very tough wood. It is compact, stable, hard, insect and mold resistant. It does not crack in conditions where other woods crack and does not mold when all woods usually rot.
But it is still a natural material and it is always better to avoid extreme conditions. It is ok if you use it at least once a week.
If you leave your house for a long period follow one of these direction:
1) fill your hinoki tub with 5" of water and leave uncovered.

2) leave a basinet containing water inside and cover the tub with its lid.

3) Wrap the exposed sides and top of the tub with clear poliethilen film. Close gaps with tape.

Avoid direct permanent sunlight exposure, expecially when not used. If you keep it always full, you might want to empty it once or twice a week and let it dry for 12 hours.