If I had to guess on a Tatum return date it would be somewhere between February 27th (the Celtics first home game after the all-star break) and March 14th (almost exactly 10 months after the surgery, but still 1 month before end of regular season).
I don't see any way he's back before the all-star break just for cautionary reasons (the all-star break is the 9-month mark) but at the same time if he's not back by mid-march or so you start running out of time to get him integrated before the playoffs. I'd also expect he's on a minutes restriction and not playing back to backs when he returns.
On a related note I think if he does come back this season I'm going to spend every minute he's on the court in a state of dread that he'll trigger some sort of lower body injury from coming back too soon. So not looking forward to that part.