Had a PHB who gave the same estimate no matter what the project was. "2 weeks." When ten days later he was asked for an ETA on the same project he'd answer "2 weeks."

I use a variety of answers myself and it is normally based on several factors. I'll ask a client "when do you need this by?" After I finish rolling around on the floor laughing hysterically at their first answer (an entire CRM application in 72 hours? REALLY?) I then negotiate something we both live with.

A trick that I'll employ is to give them a subset of features on a first pass effort and then add features over an extended period of time.

Now if it is a customer I don't like or a project I don't want to work with my estimates get padded to something ridiculous with the hopes they will get sticker shock and run away in terror. If I fail then I ratchet up my hourly rate a bit.

At that point they will either go away or I'll make more money on the project than I normally would and hopefully that will make up for the aggravation involved.

On rare instance I'll tell a client flat out to take their business elsewhere...