My answer is something like this:
I like to take on extra work and projects, but sometimes I have trouble speaking up when I've got too much on my plate and my work can suffer as a result. I've learned to ensure that I'm open about my workload and not to see it as a bad thing to tell my manager that I need assistance.
My current role required me to interview with the CEO, who asked me to name THREE weaknesses. I wasn't ready for that one!