I would say... never.
What is a tarp but something that keeps precipitation off of you. In humid summer months, sure, condensation can cause precipitation under the tarp, but in winter, this is not so much a concern, and you can pitch it lower to the ground. You might get frost inside - but just shake it off when you pack up.
Tents provide a few degrees of warmth by trapping a bit of your body heat - but that thin layer of nylon (or two) can only do so much. (And not near as much as, say, a snow cave.)
I have slept in the winter without any shelter in a -40 degree bag, and woke up with 4 inches of snow on me. I was soaked but tolerably comfortable. Had I had a tarp, it would have shed the snow off me, and I would have been perfectly fine in my 15 degree down bag (and, in point of fact, I have done this on several occasions.)
So, what is the take home message?
- If your bag is warm enough, a tarp is just the light-weight ticket to
keep you dry.
- If your bag is not warm enough, then consider a full
tent to trap those critical few degrees.
Bonus Protip: When snow camping, a tarp makes an excellent roof to a dug-out, high walled snow-fortress. Less work than a full-on snow cave.
Bonus (slightly related) Protip: A candle lantern in your tent can add a noticeable amount of warmth, and help keep things ever so slightly drier.