Mine is two years old, about twice the size. What I'm doing at the moment is watering from below (placing the pot in a shallow dish of nutrient rich water for about an hour) every two weeks or so. You'll notice a visible swelling afterwards. I've found that using this method I can kinda tell when he needs watering just from the external appearance. Apparently they should get a fair amount of sun as well, during summer, and a lot less water during winter.
The warning of course is, that after the amount of years you spend together you might never want to eat the little dude
"For as the mystic is more and more subjected to the transforming nature of the Light, he is often plunged into an acute awareness of the inadequacy and utter vileness of the lower or 'natural' self" - I.R.