Writing down everything you know seems pretty pointless. Writing down everything you fear forgetting might give you a smaller but more useful list, since it lets you cull out anything you're in no danger of forgetting (e.g. all the arithmetic facts) as well as anything you wouldn't care if you forgot (e.g. the vast majority of knowledge in your head). I actually sort of do this, in a private git repository where (alongside lists of interesting typically-paragraph-sized quotes) I keep lists of interesting (typically-sentence-fragment-sized) topic names. Sometimes the names serve as mnemonics that merely remind me of an interesting fact I once encountered but haven't thought about recently (e.g. "Rai stones"). Sometimes I'll skim through the lists and encounter a topic that I've completely forgotten about (e.g. "burying the corpse effect") and I'll quickly Google to see why I once thought it was so interesting to begin with. A little organization helps. E.g. "burying the corpse effect" was in my "economicsbits" file under the hierarchy "Financial markets, investment", "Market manipulation", "Cornering the market", so it was easy to tailor web searches to lead me to results from economists rather than morticians. I don't know if this is useful for anything more than entertainment. TimS had a very good question here.

Writing down only every arithmetic fact you know, assuming you have basic knowledge of math, would, in theory, take an infinite amount of time. In practice, the universe would end first.