Yeah, definitely have that go-bag ready. The sooner you get out the better. When I got started in the biz I had the ups and downs everybody goes through, but life was much simpler in those days, taking twice as long to get the job done isn’t that bad if it only results in being a week late. So I had some experience to look back on when I faced projects measured in months and years. This thread is important, those signs have to be read, and you have to get out before the ship hits the iceberg.
I did learn from these things well, I built my business by being the Pro from Dover called in a couple of years later after the first attempt failed. And finding the solution was usually easy, it was one of the existing simpler plans rejected initially because it didn’t satisfy a long list of arbitrary requirements. And the item so often left off the list or checked off without critical thought was “It must work”. Once so much time and money has gone down the drain the list of requirements becomes much more concise and focused.
So that is something not so easy to reduce to a simple rule, it takes the ability to assess the situation in terms of a team’s productivity potential, and the actual scope of the goal. But you can still look for the common underlying characteristic of bad projects, an initial over-reach. Beware of people who wear telescopes for glasses and think the moon is just past the tips of their fingers.