William Camden wrote:I went with three months to a year. Earlier than that, you don't have the vocabulary or basic grammar to conduct a decent conversation, unless you are taking part in some well-directed crash course.
Many people who took 2 years of French or Spanish in HS can probably start having small conversations after 3 months or so. Some, immediately.
I know there was a 10-15 year gap between HS and when I started seriously learning French again. Almost all the grammar I learned was still there. I still knew how to conjugate verbs in the Present, Passe Compose, Imparfait, and Futur. I still knew a lot of basic verbs, both Regular and Irregular (and how to Conjugate them). I still knew Pronouns, Many Prepositions. Word Order was still there. I knew how to count. How to give dates. Etc. I did a an in-person placement exam with Alliance Francaise and did incredibly well.
The only issue I had was that of phonology, which is quite normal, and vocabulary. Vocabulary is easily learned in conversational context, and actually better (and more efficiently) learned that way.
I also think that how well you learn foreign languages has something to do with your concrete grammar knowledge with your native language. Anglophones who are bad at English Grammar and aren't familiar with the language of linguistics are going to struggle more with case systems.
If someone has learned English well in HS, then all you have to do is tell them "this case is for the DO, and that case is for the IO" and they get it. Those that didn't, need to be taught the grammar of - basically - two languages at once. This is part of the reason why HS Foreign Language classes in the US tend to be ineffective these days. The French and Spanish teachers are spending half of their time going over English grammar to enable the students to understand how the grammar of those target languages work.
This becomes more and more of an issue, when you move to verb tenses, etc.
I remember studying Old and Middle English in HS English IV. We had to read Jeffrey Chaucer in the original Middle English, and Beowolf with Dual Column Old and Middle English (no Modern English translation). When you get acclimated to that, the word order differences between English and Scandinavian languages (and German, to a fairly large extent) isn't that huge of a migration. If your grammar knowledge is good, then German cases simply aren't a barrier. It significantly lightens the load IRT foreign language learning.