It depends per person. I don't know how big he is, how big the guppies are compared to him, etc. For me, personally, I'd make a slightly noticable bulge that isn't bigger than his head, if that makes sense. I don't know about other species, mine are only a few inches, but making a small bulge under the size of their head lasts them quite a while. It takes them about 3 days to digest it fully at 79 degrees.