It takes 8 planks per larder, so only 2500 larders. Best idea is to have all the planks noted in your inventory, with a lot of cash, and then use the note with what ever butler you have (I personally prefer the cook, but some people use the Demon) tell them to bring back the max amount of planks they can carry. When they bring the planks back, send them away with more while you use the ones they just brought. I like the Cook because she carries 16 planks - 2 larders - so i can manage to build and take down two in the time it takes her to fetch more. Repeat 2500 times. However, every (8 i think) runs the butler will ask for wages, otherwise they will not do anymore work. It may take a bit of time, but after a while you get into the rhythm of it. Hope this helps!