I think impeachment hearings start after Mueller's reports.
Is there any schedule when this might happen?
No, he has no deadline. It takes as long as it takes, and this investigation is likely to take a long time. Probably a few months at the minimum.
If it happens: The closer the impeachment is to the midterms, the better for Democrats. Just like Watergate.
The ideal situation for democrats would be is if there is a recommendation of adverse action sometimes next year and the economy only destabilizes the year after that.