Timers don't work on offloaded creatures. Any syndrome that ends will end immediately.
Giving an end time to syndromes with triggers like MOON_PHASE only affects the timing of the entire syndrome, which will be active or inactive based on the trigger. You can use this to, for instance, make a spell that behaves differently on different days of the month, but that's all.
What your syndrome will actually do is make it turn to stone on the full moon like a wererock, except that it will stop being a wererock as soon as it is offloaded.
The only place a long syndrome timer will be in effect is in fort mode, and that only if you play a fort continuously for 1000 in-game years.
For all practical purposes, you're better off just putting it to sleep forever. You can make it so that it is practically unkillable, but make its sweat give it a permanent unconsciousness effect. If you fight it long enough to make it tired, it will go to sleep forever. Of course, adventurers won't get any credit for this.
You can make an immobile stone Tarrasque that "wakes up" when you encounter it, perhaps with a secretion that transforms it... But if it's a megabeast, it will still go on rampages even when it's supposedly asleep, even though this makes no sense. And if it's not a megabeast you will find a whole bunch of them.
There really isn't any good way of doing this, I'm afraid. My advice is simply enjoy your Tarrasque as a generic rampaging monster and forget the hibernation.