Another kind of corollary: "If the business will go under if we don't get this done by X, then we probably need a new business plan, not faster development".

These are rules of thumb and there are definite places where they don't hold, but I've found it genuinely useful to consider when the inevitable tough questions start to get asked.

The iteration is stopped when the software has enough features and an acceptable level of bugs to be considered complete. What complete is depends entirely on the field of the software. For a proof of concept software we can stop after the first iteration, but for a safety critical software we might need 3, 4, or even more itarions.

Unfortunately, lots of us bid against people who over-promise. By the time the project is obviously behind schedule, it's too late, and the client can't switch to someone else.