This sort of thing frustrates me. There is nothing wrong with giving an estimate. If the estimate changes you can call them and amend it. This gives the dispatcher, gate agent, and hundreds of pax an idea of what is going on, let alone YOUR FELLOW COWORKER PILOT. If it the very least, you feel the need to play this "I wont tell you how long" game with crew scheds, at least have the decency to shoot a text to the other pilot so you don't waste their time as well. Courtesy. No one is getting fired over an estimate gone wrong.