GoGo, it is probably dry if it starts to develope an ashen, or light colored gray-ish appearance in the grain of the fretboard. We have an old classical guitar, the tech @ the shop showed me how to wrap nylon strings, while we had it apart, we conditioned it. It only took a few minutes, there's really no rocket science to it. Polished the frets,with a pad, being careful not to rub the wood, to remove the oxidation, and he rubbed the fretboard with, what i call a gargoyle cloth, which stays moist with the polishing oils in it's baggie. You could see the grain just magically "pop" out of the wood, and it came back to it's dark colored look. Makes a big deal in the appearance. I guess maybe a twice a year treatment would be good, but that probably depends on humidity, and the oils in your skin.