I can not say any certain number. But I can say that, to learn a language, you have to memorize sentences. It is not as hard as it seems. And I noticed that I still remember thise sentences even though I did not study them for over a year, despite the fact that I learn and speak that langauge, I do not use those specific sentencea but some words in it. For me, I wrote them in a notebook, made paper and digital flash cards and it worked.