Your first inclination when asked this question may be to respond that you don't have any downtime. However, every profession has some time when there are no immediate demands on the person's time and attention. You should prepare for this question by having 2 to 3 different activities you can perform if you're not immediately needed to monitor or fix issues related to the network.
"While it is rare, I do occasionally experience downtime when the network is running well, and there is no need to actively monitor or fix anything. During these times, I keep myself busy by updating my knowledge about network technologies, disposing of unused equipment safely and securely, networking with other members of the organization whose assistance I may need in the future, and participating in some fun team building events with my coworkers. All of these activities directly contribute to my ability to manage a high-performing and secure network infrastructure."