This is my first time planting potatoes, but from everything that I have read, you wait until you are maybe two inches taller than the next layer of your stackup. In my case, once my plants are at 8" (since these are ~6" cedar boards), I will add another box and fill her up. You want to make sure that you have good loose soil and it can freely drain. For my soil, I use a mix of sphagnum peat moss, compost, black top soil and some manure. The mix has a fair amount of sphagnum peat moss to keep the mix loose. The tators seem to be growing very well for me so far.
Here is a pic of my tato's today, just days after the picture above...