A few weeks ago, I skimmed the old HTLAL thread Slavic Language Family Learning Sequence. In the very last post, Theodisce says:
Theodisce wrote:If I was to draw a conclusion, I would say that 100 hours of listening to a language is a solid foundation upon which to build your knowledge, provided you already have a decent command of a Slavic language.
Hmm, 100 hours... Let us leave the Slavic branch for a second. Zenmonkey watches Daredevil in Portuguese. It's not unusual for Scandinavians to watch TV in their neighbouring languages. Not everyone's native language has a close relative (Modern English, anyone?), but many have a strong target language with many relatives. So, 100 hours of (somewhat?) comprehensible input, and we have a solid foundation? It's worth a try, don't you think? One hour a day for a little more than three months.
The FSI estimates are mentioned in the first post of the thread: 10 months or 44 weeks or 1100 class hours of intensive, exclusive study + about 900 hours of homework/individual study to achieve (= about 2000 total hours).
Later in the same post, ProfArguelles speaks about positive transfer:
ProfArguelles wrote:However, once one knows one language of a group, there is a considerable amount of transfer from that language to other members of the group, and Slavic languages are by all accounts + my personal experience far more closely related to each other than are either Romance or Germanic languages. Thus, while your first Slavic language may take roughly the equivalent of a year and a half of exclusive, intensive study to achieve, your second should probably take fewer than six months, your third fewer than three months, your fourth about one month, etc.
He may be right when saying that Slavic languages are particularly close to each other, I don't know. It would be interesting to see a similar (suggested) timeline (and perhaps an optimal sequence?) for other language families. Anyway, fewer than six months for a second Slavic language sounds hopeful.