I don't remember the story, but she disappears pretty quickly and then they're back to living together in 221b, so it must be pretty early in the canon.

I completely agree on the relationship thing, but I don't think John would get over Sherlock so quickly. There has to be a long grieving period, and it's not the best frame of mind to find new potential partners. I think John's depression got it covered. Especially if they plan to keep them separated only for a year, like I hope, covering the real life hyatus, and not three years.