Below is Oticon’s recommendation for long-term storage (more than 14 days). But pay equal attention to their little “Note” below that for the 12-month thing.
This implies that if you don’t use the SmartCharger, there’s still a parasitic drain on its battery. So I think it’s best to use it regularly and not just leave it unused for a long time, in the case that you also have the regular charger and opt to use the regular charger as your main charger and relegate the SmartCharger to only when you travel.
But I wouldn’t just keep it plugged in all the times either, because this will keep it at 100% state all the times, which is not good for the battery. I would use it for 3 full charges then plug it in to charge it up fully, then unplug and use it for 3 more full charges, and so on.
Technically it’s supposed to be good for 3 full charges, but I’ve read reports from users that they can squeeze maybe 5 charges out of it. So if you recharge it after every 3 full charges, you probably won’t manage to fully discharge it, so that takes care of the bottom end (don’t want it to be in a fully discharged state for too long). If you really want to take care of the top end (not to leave it in a fully charge state for too long), then try to time the full charge such that it’s done just before your end of the day so that you can drain it partially with your first full charge cycle on the hearing aids.