After Priority: Horizon, so that you get as many squadmates as possible (at least as long as Miranda survives cause otherwise it won't matter).

I would start Shore Leave way earlier than that, but if Miranda is alive the Party shouldn't be started until after Horizon. I like to run Omega right after Citadel 2, then Shore Leave. Otherwise it can feel a bit out of place taking a long vacation while the Reapers annihilate Thessia and Shepard is despondent.

I can never bring myself to complete it during the course of story. It is such a fun, uplifting final adventure with the crew, that I usually complete it after the final mission. Never felt like there was a natural break in the flow of the main quest where it fit cleanly, but if you are dead set on it occurring before the final battle, I would say close to completion of the campaign. Where enough of the prep work could have been done that some of the immediacy could have taken a break.