Scheduled maintenance downtime, or maintenance windows, are periods where the system may be unavailable while maintenance, such as upgrades, replacing parts, or installing new hardware or software. This is a scheduled event that should happen during non-peak usage hours and occur at the same time, every time. The exception to the schedule is generally only permitted for critical maintenance, and users should be notified to the greatest extent possible.
There are a couple of reasons that they are asking this question. First, they want to know that maintenance is performed during a window of time that will have the least impact on them. Second, they want to know that maintenance is performed on a regular basis so that the systems are up and available to serve them.
They expect that you have regularly scheduled maintenance windows outside of normal business hours and that these hours are known by the users of the system at the time of contracting for service or at least enough in advance that they can work around them. They may also request that the days and hours be specified in the questionnaire response.