Employers are looking for designers who can not only deliver results, but do so in a timely manner. Failing to meet your deadlines can cost your employer money or make them lose face to their customers, clients and business associates. If you are good about keeping up on your deadlines, you'll be good to go when it comes time to answer this question.

If sticking to your deadlines is something that you have a hard time with, then you need to at least show the interviewer that you respect deadlines and that you do whatever it takes to get your job done. Give examples of times when you weren't able to complete a task on deadline, and explain why you fell behind and how you rectified the situation. Did you ask for an extension ahead of time? Did you bring in another designer to help you with the work?

Keep in mind, there's no reason to punish yourself if you've missed the odd deadline here or there in the past. Potential employers want to know how you're going to handle their deadlines, so you can always turn a negative experience into something positive.

...there's no reason to punish yourself if you've missed the odd deadline here or there in the past.

Say you've missed deadlines in the past because of last minute editorial changes. You might answer the question by saying you're good at keeping deadlines so long as you have everything you need to do the job ahead of time. This way, you can answer the question positively and truthfully while also giving the employer some insight as to how you work best.