Thank you! Let's say 3500nm with 350 demand?
As explained in wapp11's link, it depends on a few different variables and they change over time. Think of the appropriateness of a plane on a scale of 0-100%. If a plane is 100% appropriate, then you get no warning and it's not problem. If the plane is 99% appropriate, you will get a warning, but realistically, you'll see no difference against competition or flying it solo. If the plane is 50% appropriate, you may suffer against competition but will be able to fill the plane on a monopoly route. If the plane is 10% appropriate, you'll be flying it empty no matter what you do.
So yes, at first, the 757 might be just fine for a few years on the route you mention, but eventually, it'll probably be a bad idea.
Ever since the introduction of the plane is too small warning, the 757 fleet has been incredibly nerfed as a fleet type, making it have no more range than a A320/B737/etc before it becomes too small, but that's a poor economic choice since those other types eat its lunch at that range. They really need a bit more leverage in the 2500-3500nm range to be more competitive.