I think the only thing we know for (relatively) sure at this point is that the Rivers of America, Tom Sawyers Island, and the train will be closing for "more than a year" starting on January 11th (and that Big Thunder Ranch is closing permanently). Other than that, I haven't seen anything that gives a firm date about when they expect Star Wars land to be completed.
In terms of the Rivers of America, I don't think it'll be drained for the entire construction period because that would mean taking F! out of service for several years. Still, it does sound like it could be empty for at least a year (2016) and potentially beyond.