1. Never give an estimate in the same conversation in which it was requested, unless you're quite sure the answer is "less than an hour". Especially if they are going to turn around and quote your timeframe to a customer. The reason for this rule is:
2. The more you think about an estimate, the higher it will go. "Oh yeah and we'll also have to..." is 100x more common than "I found a shortcut that doesn't compromise on..."
3. Even if they know it's an estimate and not a deadline, they might not know the difference between "40 hours of dedicated effort" and "One week's worth of business hours". Making that distinction is just as important as the estimate/deadline distinction.
I'm lucky to work where we don't spend much effort on estimates and time tracking, both of which just make things take longer and cause productivity-reducing stress.