Fortunately (or unfortunately), editors set our deadlines for us, but occasionally, an editor will ask, "When do you think you can turn this in?" Honesty is always the best policy, even if you are just saying "I don't think I have time to do this" (an editor will respect that more than you stringing them along or turning in shoddy work.) WorldWit has some more thoughts on the importance of choosing your words wisely when promising a product.