It depends on what you consider good, but generally it is possible to become a a proficient enough salsa dancer in maybe 2 or 3 month, if you are keen and work on it. Importantly, many (most) salsa courses focus too much on moves and not enough on musicality and the basics (also due to student demand). I would argue you can have great fun with the basic food work and 2-3 moves, and I know that most followers really enjoy the dance even if the vocabulary is limited, as long as there is rhythm. This applies to Salsa in general, but LA/linear is more focused on moves and Cuban more on musicality I would say.