The main "problem" is that offering Visual Diffs to editors who use VisualEditor should increase feedback, and that feedback needs to be turned into reports for the devs in a timely fashion.
We know when we're done with this task when the initial feedback tapers off, and we move this product to normal/ongoing support. However: Visual Diffs is going to have several phases of "beta" status:
the current one, which is a new feature inside VisualEditor (no opt-out, but you don't have to click the button to use it if you don't want to),
the next one (presented as the initial option in the visual editor, and you have to manually switch to wikitext diff), and
a separate Beta Feature for "historical" diffs (e.g., using it in Special:RecentChanges).
This task should only cover the first two. Support for historical diffs should be handled in a separate task.