Pharmacy mineral oil works fine but needs renewed every few months. I
> usually pour a small amount on the board, rub it around with my bare hands
> and blot/wipe it lightly with a soft, lint free rag (t-shirt, sheet,
> etc.). Pretty similar to tung or danish oil application. If you let the
> wet oil set on the board it will take forever to dry.
>
Or, in the case of mineral oil - never. That's the weakness. It collects dirt and microbes in its sticky grip and makes it difficult for polar solvents like water, even with the help of surfactants, to get to 'em.