I live in an arid environment and have never worried about leaving filament out for an extended period of time. To reduce how frequently we receive static shocks I installed a whole house humidifier with a target humidity of 35 %. Is this enough humidity to warrant storing filament in a dry box? At what point does ambient humidity become a concern?
The closer to zero humidity the better. Maybe a garage or some place protected from rare precipitation outside would be better. If one room is considerably less humid than the others, you could use it. Keep the bags and dry packs the filament comes in and put them back in when not in use.