Somebody someplace has already solved this problem (or one very similar to it.)
Creativity is now finding that solution and adapting it to this particular problem. — The TRIZ Journal
So when you hear we've never done this before and we can't possibly estimate how long it will take, think of this quote and don't think for a minute that those using that excuse have any basis to stand on. Go find someone who knows how to solve problems in the presence of uncertainty.