I would go to a craft store and get some of the gel plant things..it is like little balls that you fill with water, or nute water, and then just put them in the soil...they hold the water til the soil around it pulls the moisture out...it would take probably a couple of weeks in between each watering once you add those things though, simply because everytime you did water it would be absorbed by the little balls again and then they would water for you til dry again...anyway they are a good idea for any extended time I think...they can't fail, just put them under a layer of soil so the water doesn't just evaporate...
Have taken a vacation with using these in soil and coco and come home to awesome plants...