Not a silly question at all. The answer is once you have as much debris as you want, which should be no more than half the height in the bed you are filling, you can fill it with soil and plant in it straight away. With hügelculture, the debris will rot making good compost in the bed, which means as it rots the soil level will go down and you will have to top it off occasionally with soil or compost. The beds we built this year and planted in have sunk a good three inches in three months. I won't top it off until I'm ready to pull out the tomatoes and things I have growing in there and am ready to prep the bed for the next crop.
This pic is early in the season so these tomatoes are enormous now. This bed is about 2 1/2 feet high for me.
