I don't have muscovies, but from what I understand, daylength affects laying. Some breeds, however, do seem to lay a bit even in winter. This is my first year with runners, and I expect their laying will taper off (I live in New England), but I have yet to see how it goes.
I would anticipate the egg laying would drop off, but it would depend on how well they eat, also, I think, during winter - I think of them as southern birds, so I wonder if their bodies would spend energy they would otherwise use developing eggs just to stay warm. I hope a northern muscovy keeper can weigh in here!