It comes back with a little exposure to the language. I find I can keep up my proficiency in Russian, which I spent a year studying intensively 44 years ago when I was in the army, by reading a book every year or two, even though I have little opportunity to speak the language. Reading brings back the vocabulary and idioms; I don't seem to have much trouble with the grammar. For modern languages, you might try spending a few minutes a day keeping up with the news on the internet. I can't speak for Sumerian or Akkadian, though.