You should never pretend you don't have any nor come up with a list like "I work too much." Come up with a real weakness, like prioritization, time management perhaps - something that is real but also wouldn't jeopardized your ability to do the job and focus on what you do about it. I'm working on improving my time management for example, I use an online calendar and schedule my tasks inside that calendar and plan my week each Monday estimating time needed for each task and project.