And there's always the possibility that if you announce too late competitors will eat your lunch.
One way to handle this is to spend some time on a formal estimate. Two to four weeks of R&D to scope a project can help narrow estimates to something approaching realism. You'll still be wrong, but you're less likely to be hopelessly wrong.
Asking someone for an instant opinion is madness. That's not an informed estimate, it's just a guess, and usually worthless.