We don’t expect to hear anything from the studio anytime soon, as one of the most characteristic features of Disney is that they tend to be very quiet about their work until the very end.
When they start speaking about “Frozen 2” on the official front, this would likely mean that the film is almost ready to be released, so fans should keep an eye out for such statements next year.
It’s most likely not realistic to expect any news about the film during this year, as there are various hints that indicate that filming hasn’t even started yet. The statements of Idina Menzel, for example, are a clear indicator that the script isn’t finished at this stage, or at least it hasn’t been handed out to the key actors in the production.
It’s important to remember that this is an animated film though, and things tend to work a bit differently with those.
Voice acting can usually be done at the final stages of production, after the animation has already been completed. This makes it possible to keep actors in the dark until the very end, while also keeping things very flexible for the animators, who have complete freedom in rearranging scenes, dialog, and so on.
Knowing this, a statement from Idina Menzel about the nature of her role would likely indicate that the film is already in an advanced stage of its production.