If they want to reveal Chrom with marketing in mind, I see him coming very soon or very late.
Very soon if they still think it could influence Awakening sales any notable amount. Kind of missed the window.
Very late if they want to show him off (or Marth?) after a promotional trailer for SMTxFE on Nintendo Direct, likely with a
advertising the game at the bottom of the page.
Of course they could not reveal him at all before release or not with any regard to marketing, though just outlining some possibilities. We don't know how far along development it is, but if Atlus is all involved, they generally show games closer to release than other companies, even if Nintendo is the publisher.